Cling (6)
There are a few possibilities for this:
hold on
'cling' is the definition.
(I know that cling can be written as cohere)
This is all the clue.
(Other definitions for cohere that I've seen before include "Stick together, remain united" , "Hold firmly together" , "Even; without jerks" , "Fuse together" , "Stick or hold together and resist separation" .)
'cling' is the definition.
(synonyms)
This is all the clue.
(Other definitions for adhere that I've seen before include "Header (anag)" , "Stick like a stamp" , "Stick to or continue faithful" , "Stick to as by suction" , "Stick to, cling" .)
'cling' is the definition.
(I know that cling is a more specific form of hold on)
This is the entire clue.
(Other definitions for hold on that I've seen before include "Keep one's grasp or maintain telephone contact" , "Informally, just a mo', or don't let go" , "Cling to or informally stop a mo'" , "Retain grasp, and don't ring off" , "Cling to - or stop right there" .)