Old unionist set off in a canoe (9)
I believe the answer is:
outrigger
'canoe' is the definition.
(outrigger canoe is a kind of canoe)
'old unionist set off' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'unionist' becomes 'u' (abbreviation as in Democratic Unionist Party).
'set off' becomes 'trigger' (synonyms).
'o'+'u'+'trigger'='OUTRIGGER'
'in a' acts as a link.
(Other definitions for outrigger that I've seen before include "Boat stabiliser" , "Git, our Reg (anag.); boat" , "Ship's stabilising float" , "Canoe with stabilisers" , "Projecting stabiliser" .)