Couple allowed to join band (8)
I believe the answer is:
bracelet
'band' is the definition.
(bracelet is a kind of band)
'couple allowed to join' is the wordplay.
'couple' becomes 'brace' ('brace' can be a synonym of 'couple'**).
'allowed' becomes 'let' (I've seen this before).
'to join' means one lot of letters go next to another.
'brace'+'let'='BRACELET'
(Other definitions for bracelet that I've seen before include "Ornament - handcuff (slang)" , "Ornament worn on arm or wrist" , "Arm band" , "Rebel cat (anag.)" , "Jewellery worn around the wrist" .)