Join up with Len Turner first (6)
I believe the answer is:
enlist
'join up' is the definition.
(I've seen this before)
'with len turner first' is the wordplay.
'with' says to put letters next to each other.
'turner' is an anagram indicator (letters must turn into another order).
'first' becomes 'ist'.
'len' anagrammed gives 'enl'.
'enl'+'ist' is 'ENLIST'.
(Other definitions for enlist that I've seen before include "Enrol in the services" , "Join up, sign on" , "Sign on -- 13 (anag)" , "Draft" , "Register" .)