Understand regressive part of UKIP's arguments (5)
I believe the answer is:
grasp
'understand' is the definition.
(I know that grasp is a more specific form of the action the understand)
'regressive part of ukip's arguments' is the wordplay.
'regressive' is a reversal indicator (to regress is to go backwards).
'part of' indicates the answer is hidden within the clue.
'psarg' is hidden in the letters of 'ukips arguments'.
'psarg' in reverse letter order is 'GRASP'.
(Other definitions for grasp that I've seen before include "Comprehension" , "Seize, hold firmly" , "Take hold of firmly" , "Support; rugby player" , "Hold - understanding" .)