Men keeping quiet concerning trees (6)
I believe the answer is:
maples
'trees' is the definition.
(maple is a kind of tree)
'men keeping quiet' is the wordplay.
'men' becomes 'males' (I've seen this in another clue).
'keeping' is an insertion indicator (some letters kept inside).
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'males' placed around 'p' is 'MAPLES'.
'concerning' is the link.
(Other definitions for maples that I've seen before include "Trees of Canada" , "Trees providing syrup" , "MP sale of trees" , "Acers" , "(Canadian) trees" .)