Bear with head cut off? I say! (3)
I believe the answer is:
ooh
'i say' is the definition.
(both are expressions of surprise or shock)
'bear with head cut off?' is the wordplay.
'bear' becomes 'pooh' (Winnie the Pooh).
'with head cut off?' means to remove the first letter.
'pooh' with its initial letter taken off is 'OOH'.
(Other definitions for ooh that I've seen before include "expression of delight" , "Exclamation of surprise or pain" , "Expression of surprise" , "sound excited" , "Exclamation of delight" .)