Irishman back in Knock (3)
I believe the answer is:
tap
'knock' is the definition.
(I know that knock can be written as tap)
'irishman back' is the wordplay.
'irishman' becomes 'pat'.
'back' says the letters should be written in reverse.
'pat' in reverse letter order is 'TAP'.
'in' is the link.
(Other definitions for tap that I've seen before include "Valve controlling flow - light knock" , "Flow controller; knock" , "Dancing" , "Secret (phone) intercept" , "Obtain money from" .)