Join up forming lines on time (6)
I believe the answer is:
enlist
'join up' is the definition.
'forming lines on time' is the wordplay.
'forming' indicates anagramming the letters.
'on' means one lot of letters go next to another.
'time' becomes 't' (**).
'lines' with letters rearranged gives 'enlis'.
'enlis'+'t'='ENLIST'
(Other definitions for enlist that I've seen before include "Join the armed services" , "Listen (anag.)" , "Join the armed forces" , "Draft" , "augment army" .)