Join up squiggly lines, ending in portrait (6)
I believe the answer is:
enlist
'join up' is the definition.
'squiggly lines ending in portrait' is the wordplay.
'squiggly' is an anagram indicator.
'ending in' suggests the final letters.
The last letter of 'portrait' is 't'.
'lines' with letters rearranged gives 'enlis'.
'enlis'+'t'='ENLIST'
(Other definitions for enlist that I've seen before include "Take the Queen's shilling" , "Join up, sign on" , "Draft" , "Register" , "Enrol in the services" .)