Hickory (chicory - C + K)
Ahoy! Need oxygen! Jerk! (5)
Yahoo (ahoy + O)
+++++
Piglet called Pooh back, then he called Hal back (as if it was a big deal). (7)
HOOPLAH (pooh backwards + hal backwards)
Up there is one skull. (5)
Up there is one skull. (5)
AHEAD (A + Head)
++++++
Vice President trashed valentine — so sad. (5, 9)
ADLAI STEVENSON (valentine + so + sad)
Mixed red peppers provided by favorite relative.
How many letters?
Mixed red peppers provided by favorite relative. (8)
Nepotism (anagram of PIMENTOS)
++++++
He’ll let you call him that to his face, but through emails? Fucked up. (6)
ETA: Happy Dope birthday to me! 
Yay! kaylasdad99! Yay! 
ISMAEL (emails anagram)
Happy Birthday!
Soccer legend returned with Harrison in time for white sale item. (8)
ELEPHANT (PELE backwards + HAN + T)
+++++
Steve McGarret dug through the endless uranium, and found a little boy flower. (9)
Anthurium?
Correct.
How is it Anthurium?
I wasn’t sure either. The ANT comes from the overlap, and the IUM from “endless [the front end I guess] uranium”. But unclear to me where the HUR in the middle comes from. Otherwise it fit the clue, hence the ?.
Nevertheless,
Careful with that axe, mumu and gamma ray, Eugene. (9)
Anagram of URANIUM + TH. (“the” endless is “th”) When you dig through that, you mix up the letters. “Steve McGarret” was an extra hint to point you at Hawaii, where anthuriums are called “little boy flowers.”
Nice 
In play:
Careful with that axe, mumu and gamma ray, Eugene. (9)
Careful with that axe, mumu and gamma ray, Eugene. (9)
UMMAGUMMA (anagram mumu, gamma)
+++++
Alarming activity along the border. (8)
marginal (anagram)
+++++
Nan dissolves with glee at the idea of plastic lab equipment (7)
NALGENE (nan + glee)
Rock star rearranges immense song (4, 7)