For one Guardian leader on top teacher, it’s intellectual (7)
I believe the answer is:
egghead
'it's intellectual' is the definition.
(egghead is a kind of intellectual)
'for one guardian leader on top teacher' is the wordplay.
'for one' becomes 'eg' (both can mean 'for example').
'leader' says to take the initial letters.
'on' says to put letters next to each other.
'top teacher' becomes 'head' (headteacher).
The initial letter of 'guardian' is 'g'.
'eg'+'g'+'head'='EGGHEAD'
(Other definitions for egghead that I've seen before include "Boffin" , "scholar" , "Brainy type" , "where soldiers may take a dip?" , "Intellectual (colloq.)" .)