Former Tory leader on engineers - fanatical! (7)
I believe the answer is:
extreme
'fanatical' is the definition.
(thesaurus)
'former tory leader on engineers' is the wordplay.
'former' becomes 'ex'.
'tory leader' becomes 't' (I've seen this in other clues).
'on' says to put letters next to each other.
'engineers' becomes 'reme' (I've seen this in other clues).
'ex'+'t'+'reme'='EXTREME'
(Other definitions for extreme that I've seen before include "stiff" , "Acute, maximum" , "Anything but moderate" , "not least" , "Far beyond the norm" .)