Irish copper in helmet? Could be Bob (7)
I believe the answer is:
haircut
'be bob' is the definition.
(bob is a type of haircut)
'irish copper in helmet?' is the wordplay.
'irish' becomes 'IR' (abbreviation).
'copper' becomes 'cu' (chemical symbol).
'in' means one lot of letters goes inside another.
'helmet?' becomes 'hat' (helmet is a kind of hat).
'ir'+'cu'='ircu'
'ircu' inserted into 'hat' is 'HAIRCUT'.
'could' acts as a link.
(Other definitions for haircut that I've seen before include "style for shock" , "Short back and sides?" , "Style of coiffure" , "Tonsorial style" , "Less of a shock after this" .)