Cling to – had come loose before (6)
I believe the answer is:
adhere
'cling to' is the definition.
(I've seen this before)
'had come loose before' is the wordplay.
I cannot really understand how this works, but
an anagram of 'had' is 'adh' which is present in the answer.
'before' could be 'ere' (usually literary) and 'ere' is found within the answer.
No letters remain.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for adhere that I've seen before include "Stick to, cling" , "'Stick to, a play say (6)'" , "Stick to as by suction" , "Remain attached" , "Keep" .)