Couple starting to work out (3)
I believe the answer is:
two
'couple' is the definition.
(two people or things)
'starting to work out' is the wordplay.
'starting' indicates taking the first letters.
The first letters of 'to work out' is 'TWO'.
(Other definitions for two that I've seen before include "A pair (3)" , "Number of faces of the god Janus" , "Even number" , "'It takes ? to tango'" , "A low number" .)