Blow up, making fat with line (7)
I believe the answer is:
inflate
'blow up' is the definition.
(synonyms)
'making fat with line' is the wordplay.
'making' indicates anagramming the letters.
'with' is a charade indicator (letters next to each other).
'fat' put after 'line' is 'linefat'.
'linefat' is an anagram of 'INFLATE'.
(Other definitions for inflate that I've seen before include "Dilate" , "Blow or pump up" , "Blow up like balloon" , "Distend" , "Enlarge" .)