Trees ahead of tense maiden (5)
I believe the answer is:
first
'maiden' is the definition.
(synonyms)
'trees ahead of tense' is the wordplay.
'trees' becomes 'firs' (fir is a kind of tree).
'ahead of' means one lot of letters go next to another (some letters go in front of others).
'tense' becomes 't' (abbreviation used in many dictionaries).
'firs'+'t'='FIRST'
(Other definitions for first that I've seen before include "in dominant position" , "Earliest; good degree" , "Preliminary" , "Rifts (anag)" , ""Eclipse _____ ..." .)