Understand some of polemic at church (5)
I believe the answer is:
catch
'understand' is the definition.
(I know that catch is a more specific form of the action the understand)
'some of polemic at church' is the wordplay.
'some of' indicates the answer is hidden within the clue.
'CATCH' is hidden within 'polemic at church'.
(Other definitions for catch that I've seen before include "Snag - trap" , "Desirable husband; unforeseen problem" , "Discover" , "Hidden problem; trap" , "Fastener on door or window" .)