Ace taking some exercise to get fit (3)
I believe the answer is:
apt
'fit' is the definition.
(apt means appropriate or fitting)
'ace taking some exercise' is the wordplay.
'ace' becomes 'a' (abbreviation).
'taking' means one lot of letters go next to another.
'some exercise' becomes 'pt' (Physical Training).
'a'+'pt'='APT'
'to get' is the link.
(Other definitions for apt that I've seen before include "Pertinent" , "Tending" , "Tap (anag.)" , "Relevant" , "Suitable for Pat" .)