Chummy's apt to be tight (5-7)
I believe the answer is:
close-fitting
'be tight' is the definition.
('close-fitting' can be similar in meaning to 'tight')
'chummy's apt' is the wordplay.
'chummy' becomes 'close' ('close' can be similar in meaning to 'chummy').
'apt' becomes 'fitting' (I've seen this before).
'close'+'fitting'='CLOSE-FITTING'
'to' is the link.
(Other definitions for close-fitting that I've seen before include "Designed to be worn tightly" , "Figure-hugging" , "Designed to hug the figure" .)