Push forward in short formal line (6)
I believe the answer is:
propel
'push forward' is the definition.
(to propel is to push forward)
'in short formal line' is the wordplay.
'in short' means to remove the last letter.
'formal' becomes 'proper' (I've seen this before).
'line' becomes 'l' (used when specifying particular lines from a poem).
'proper' with its final letter removed is 'prope'.
'prope'+'l'='PROPEL'
(Other definitions for propel that I've seen before include "Give impetus to" , "Derive of push forward" , "Drive or push forward as with oars" , "Cause to move forward with force" , "Launch" .)