Join up squiggly lines, straight finally (6)
I believe the answer is:
enlist
'join up' is the definition.
(I've seen this in another clue)
'squiggly lines straight finally' is the wordplay.
'squiggly' indicates anagramming the letters.
'finally' indicates one should take the final letters.
The last letter of 'straight' is 't'.
'lines' is an anagram of 'enlis'.
'enlis'+'t'='ENLIST'
(Other definitions for enlist that I've seen before include "Enrol in armed forces" , "Register" , "Sign on -- 13 (anag)" , "Join the army" , "augment army" .)