Not short, line concerning head of government (4)
I believe the answer is:
long
'not short' is the definition.
('long' is the opposite to 'short')
'line concerning head of government' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'concerning' becomes 'on'.
'head of' says to take the initial letters.
The initial letter of 'government' is 'g'.
'l'+'on'+'g'='LONG'
(Other definitions for long that I've seen before include "summer break?" , "Sustained" , "Lengthy - pine" , "bought in advance" , "dream" .)