Idiotic Head of Accounts is retiring before watershed? (7)
I believe the answer is:
asinine
'idiotic' is the definition.
(both can mean foolish or silly)
'head of accounts is retiring before watershed?' is the wordplay.
'head of' says to take the initial letters.
'retiring' shows that the letters should be reversed in order.
'before' means one lot of letters go next to another.
'watershed?' becomes 'nine' (nine o'clock is the UK TV watershed).
The initial letter of 'accounts' is 'a'.
'is' written backwards gives 'si'.
'a'+'si'+'nine'='ASININE'
(Other definitions for asinine that I've seen before include "Extremely stupid or foolish" , "Silly like a donkey" , "Idiotic" , "Stupid (as a donkey?)" , "Jenny's often like this" .)