Think again about swilling cider, son! (10)
I believe the answer is:
reconsider
'think again' is the definition.
('consider' is an answer for 'think' so perhaps 'reconsider' is OK for 'think again')
'about swilling cider son' is the wordplay.
'about' becomes 're' (common abbreviation for 'with reference to').
'swilling' is an anagram indicator (swill can mean to stir or swirl round).
'cider'+'son'='ciderson'
'ciderson' anagrammed gives 'consider'.
're'+'consider'='RECONSIDER'
(Other definitions for reconsider that I've seen before include "Nice orders ( anag.)" , "Have second thoughts" , "Think again" , "Ponder further" .)