Is able to hold on as a rule (5)
I believe the answer is:
canon
'a rule' is the definition.
(canon is a kind of rule)
'is able to hold on' is the wordplay.
'is able to' becomes 'can' (if you can do something you are able to do it).
'hold' means one lot of letters go next to another.
'can'+'on'='CANON'
'as' is the link.
(Other definitions for canon that I've seen before include "General rule; clergyman" , "Body of works" , "in the ministry" , "Standard" , "Moron" .)