Welshman working we must interrupt (4)
I believe the answer is:
owen
'welshman' is the definition.
(I've seen this before)
'working we must interrupt' is the wordplay.
'working' becomes 'on'.
'must interrupt' indicates putting letters inside.
'on' placed around 'we' is 'OWEN'.
(Other definitions for owen that I've seen before include "Welsh industrialist and social reformer, founder of cooperative communities, d. 1858" , "English war poet, killed 1918" , "Wilfred -, English poet killed in World War One" , "Former SDP leader" , "'E won over the Welshman" .)