Underground worker arresting a Republican sailor (7)
I believe the answer is:
mariner
'sailor' is the definition.
(mariner is a kind of sailor)
'underground worker arresting a republican' is the wordplay.
'underground worker' becomes 'miner' (I've seen this before**).
'arresting' means one lot of letters goes inside another.
'republican' becomes 'r' (abbreviation).
'a'+'r'='ar'
'miner' placed around 'ar' is 'MARINER'.
(Other definitions for mariner that I've seen before include "Seafarer, old salt" , "Bluejacket" , "Seaman" , "Person sailing" , "Sailor" .)