Join up, using tinsel (6)
I believe the answer is:
enlist
'join up' is the definition.
'using tinsel' is the wordplay.
'using' indicates anagramming the letters.
'tinsel' with letters rearranged gives 'ENLIST'.
(Other definitions for enlist that I've seen before include "Enrol in armed forces" , "Take the Queen's shilling" , "Join the armed forces" , "Join up - tinsel (anag)" , "Enrol - recruit" .)