Son given rebuke is down (4)
I believe the answer is:
Here is my best explanation:
'is down' is the definition.
'son given rebuke' is the subsidiary indication.
'son' becomes 's'.
'given' says to put letters next to each other.
'rebuke' becomes 'wig' (wig is a kind of rebuke).
(Other definitions for swig that I've seen before include "Gulp of drink", "Quick drink", "Swallow (drink) greedily", "(Take) quick drink from bottle", "Drink (a deep draught)", "Large swallow", "Glug", "Drink deeply", "Drink hurriedly" and "Swallow drink hurriedly".)