Working to accommodate exercise that's free (4)
I believe the answer is:
open
'free' is the definition.
(similar in meaning)
'working to accommodate exercise' is the wordplay.
'working' becomes 'on' (on can mean working or activated).
'to accommodate' is an insertion indicator.
'exercise' becomes 'pe' (Physical Education).
'on' enclosing 'pe' is 'OPEN'.
'that's' is the link.
(Other definitions for open that I've seen before include "See 12" , "Ready for business; spread out; manifest" , "Candid and undisguised" , "Release" , "Available for business, exposed" .)