Silent guy climbing with relations (5)
I believe the answer is:
tacit
'silent' is the definition.
(I know that silent can be written as tacit)
'guy climbing with relations' is the wordplay.
'guy' becomes 'cat' (American slang for man, esp in Jazz).
'climbing' shows that the letters should be reversed in order (in a down clue, letters climb up).
'with' is a charade indicator (letters next to each other).
'relations' becomes 'it' (both can mean sex).
'cat' written backwards gives 'tac'.
'tac'+'it'='TACIT'
(Other definitions for tacit that I've seen before include "Attic? (anag)" , "Unspoken but implied" , "Understood without being directly expressed" , "Understood, implicit" , "Implied but not directly expressed" .)