Dress up in small, small number (7)
I believe the answer is:
sweeten
'dress up' is the definition.
(thesaurus)
'small small number' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'small' becomes 'wee' ('wee' can be similar in meaning to 'small').
'number' becomes 'ten' (ten is a kind of number).
's'+'wee'+'ten'='SWEETEN'
'in' is the link.
(Other definitions for sweeten that I've seen before include "Add sugar or honey" , "Soften up" , "Make fresh" , "Seen wet (anag.)" , "Make sugary or saccharine" .)