Part of cap that is fitting (3)
I believe the answer is:
apt
'is fitting' is the definition.
(I've seen this in other clues)
'part of cap that' is the wordplay.
'part of' says the answer is hidden in the clue.
'APT' can be found hidden inside 'cap that'.
(Other definitions for apt that I've seen before include "Fitting - inclined" , "Inclined; appropriate" , "apartment in Queens, say" , "Fit, suitable" , "suitable for the circumstances" .)