Write gym plan at last (3)
I believe the answer is:
pen
'write' is the definition.
(I've seen this before)
'gym plan at last' is the wordplay.
'gym' becomes 'pe'.
'at last' indicates one should take the final letters.
The last letter of 'plan' is 'n'.
'pe'+'n'='PEN'
(Other definitions for pen that I've seen before include "Writing implement" , "Shut up" , "International Association of poets, playwrights etc" , "Write" , "Pound" .)