Upset with head measurement? (7)
I believe the answer is:
capsize
'upset' is the definition.
(I've seen this before)
'with head measurement?' is the wordplay.
'with' means one lot of letters go next to another.
'head' becomes 'cap' (both can mean the top of something).
'measurement?' becomes 'size' (size is a kind of measurement).
'cap' next to 'size' is 'CAPSIZE'.
(Other definitions for capsize that I've seen before include "Upset" , "Turn so that keel faces upwards" , "Overturn (a boat)" , "Of boat, turn turtle" .)