Prime time under trees (5)
I believe the answer is:
first
'prime' is the definition.
(similar in meaning)
'time under trees' is the wordplay.
'time' becomes 't' (abbreviation).
'under' means one lot of letters go next to another (in a down clue, some letters go below others).
'trees' becomes 'firs' (fir is a kind of tree).
't' after 'firs' is 'FIRST'.
(Other definitions for first that I've seen before include "Top university degree" , "second to none" , "Top degree class" , "Most important" , "Foremost" .)