Like a schoolmaster on holiday - not at his best (3,2,4)
I believe the answer is:
out of form
'like a schoolmaster' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'on holiday not at his best' is the wordplay.
I cannot quite see how this works, but
'not' could be 'o' (I've seen this in other clues) and 'o' is found in the answer.
'on holiday' could be 'off' (I've seen this in another clue) and 'off' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for out of form that I've seen before include "not playing well" , "Off school" , "In poor shape" .)