Fitting a small part (3)
I believe the answer is:
apt
'fitting' is the definition.
(both mean appropriate)
'a small part' is the wordplay.
'small part' becomes 'pt' (abbreviation; 'small' indicates a short form).
'a'+'pt'='APT'
(Other definitions for apt that I've seen before include "Suitable, appropriate" , "To the point" , "Fitting - inclined" , "Fit, suitable" , "suitable for the circumstances" .)