Pacify son, very small, just into double figures (7)
I believe the answer is:
sweeten
'pacify' is the definition.
(thesaurus)
'son very small just into double figures' is the wordplay.
'son' becomes 's'.
'very small' becomes 'wee' ('wee' is a Scottish word for small).
'just into double figures' becomes 'ten' (10 is the first age with double figures).
's'+'wee'+'ten'='SWEETEN'
(Other definitions for sweeten that I've seen before include "placate" , "Appease" , "Make saccharine or honeyed" , "Seen wet (anag.)" , "Add sugar or honey" .)