Arrest an alternative bishop (3)
I believe the answer is:
nab
'arrest' is the definition.
('nab' can be a synonym of 'arrest')
'an alternative bishop' is the wordplay.
'alternative' indicates an anagram.
'bishop' becomes 'b' (abbreviation used in chess).
'an' anagrammed gives 'na'.
'na'+'b'='NAB'
(Other definitions for nab that I've seen before include "Pinch" , "Catch (a felon)" , "Apprehend" , "nail" , "Cop" .)