"Trees" with T at the front! (5)
I believe the answer is:
first
'the front' is the definition.
(thesaurus)
'trees with t' is the wordplay.
'trees' becomes 'firs' (fir is a kind of tree).
'with' says to put letters next to each other.
'firs'+'t'='FIRST'
'at' is the link.
(Other definitions for first that I've seen before include "Leading" , "The one that precedes all others, in time say" , "to begin with" , "second to none" , "Before all else" .)