The man in Ring keeps key that opens entrance rattling in jacket (6)
I believe the answer is:
reefer
'jacket' is the definition.
(thesaurus)
'the man in ring keeps key that opens entrance rattling' is the wordplay.
I cannot really understand how this works, but
'key' could be 'reef' (I've seen this before) and 'reef' is found in the answer.
The remaining letters 'er' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'in' acts as a link.
(Other definitions for reefer that I've seen before include "Type of jacket - smoke" , "Joint; spliff" , "refrigerated transport" , "drug-filled cigarette" , "Hand-rolled cigarette" .)