The way to get a six-pack? It's in drinks (3-3)
I believe the answer is:
sit-ups
'the way to get a six-pack?' is the definition.
'it's in drinks' is the wordplay.
'it's' becomes 'it' (I've seen this in other clues).
'in' indicates putting letters inside.
'drinks' becomes 'sups' (I've seen this before**).
'it' put within 'sups' is 'SIT-UPS'.
(Other definitions for sit-ups that I've seen before include "Gym exercises" , "Tummy tighteners" , "Physical training exercises" .)