Defenceless Unionist called about Republican (7)
I believe the answer is:
unarmed
'defenceless' is the definition.
(similar in meaning)
'unionist called about republican' is the wordplay.
'unionist' becomes 'u' (abbreviation as in Democratic Unionist Party).
'called' becomes 'named' ('name' can be a synonym of 'call').
'about' means one lot of letters goes inside another.
'republican' becomes 'r' (abbreviation).
'named' placed around 'r' is 'narmed'.
'u'+'narmed'='UNARMED'
(Other definitions for unarmed that I've seen before include "Type of combat" , "Like Venus de Milo" , "Defenceless" , "Lacking weapons" , "Having no means of defence" .)