Gently supporting son, not very much (8)
I believe the answer is:
slightly
'not very much' is the definition.
(I've seen this before)
'gently supporting son' is the wordplay.
'gently' becomes 'lightly' ('lightly' can be a synonym of 'gently').
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'son' becomes 's'.
'lightly' after 's' is 'SLIGHTLY'.
(Other definitions for slightly that I've seen before include "To a small degree" , "Not very much" , "little bit" , "To a small extent" , "a tiny bit" .)