Join up with Len Turner first (6)
I believe the answer is:
enlist
'join up' is the definition.
(I've seen this in another clue)
'with len turner first' is the wordplay.
'with' is a charade indicator (letters next to each other).
'turner' is an anagram indicator (letters must turn into another order).
'first' becomes 'ist' (ist resembles 1st).
'len' is an anagram of 'enl'.
'enl' put next to '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" .)