Mostly stick together to protect single or joint beneficiary (6)
I believe the answer is:
coheir
'joint beneficiary' is the definition.
The definition suggests a singular noun which matches the answer.
'mostly stick together to protect single' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'stick together' becomes 'cohere' (to cohere is to stick or fit together).
'to protect' is an insertion indicator (inserted letters are protected or taken in).
'single' becomes 'I' (I as in one).
'cohere' with its last letter taken off is 'coher'.
'coher' enclosing 'i' is 'COHEIR'.
'or' is the link.
Can you help me to learn more?
(Other definitions for coheir that I've seen before include "one of several in succession?" , "Not the only one to succeed" .)