Joins up tinsel somehow before the end of Christmas (7)
I believe the answer is:
enlists
'joins up' is the definition.
(I've seen this before)
'tinsel somehow before the end of christmas' is the wordplay.
'somehow' indicates an anagram.
'before' is a charade indicator (letters next to each other).
'the end of' suggests the final letters.
The final letter of 'christmas' is 's'.
'tinsel' is an anagram of 'enlist'.
'enlist'+'s'='ENLISTS'
(Other definitions for enlists that I've seen before include "gets involved" , "Registers" , "Listens (anag.)" , "volunteers" , "Recruits" .)