Head of school and the writer accepting satisfactory fag (5)
I believe the answer is:
smoke
'fag' is the definition.
(thesaurus)
'head of school and the writer accepting satisfactory' is the wordplay.
'head of' suggests taking the first letters.
'and' means one lot of letters go next to another.
'the writer' becomes 'me' (the person who wrote the clues).
'accepting' is an insertion indicator.
'satisfactory' becomes 'ok' (similar in meaning).
The initial letter of 'school' is 's'.
'me' enclosing 'ok' is 'moke'.
's'+'moke'='SMOKE'
(Other definitions for smoke that I've seen before include "Cure (meat) over fire" , "Use tobacco" , "Use a cigarette" , "Carbon vapour" , "Use cigarettes" .)