School head's vain — creep! (5)
I believe the answer is:
sidle
'creep' is the definition.
(I've seen this before)
'school head's vain' is the wordplay.
'head's' says to take the initial letters (I've seen 'heading' mean this (head could mean the top letter)).
'vain' becomes 'idle' (I've seen this in another clue).
The first letter of 'school' is 's'.
's'+'idle'='SIDLE'
(Other definitions for sidle that I've seen before include "'Move sideways, furtively perhaps (5)'" , "Slide (anag.)" , "Walk stealthily" , "Isled (anag.)" , "Make cautious progress" .)