Starts to believe Liberal's acting honourably? Nonsense! (4)
I believe the answer is:
blah
'nonsense' is the definition.
(I have seen 'Pretentious nonsense' mean 'blah' so perhaps 'nonsense' could also mean 'blah')
'starts to believe liberal's acting honourably?' is the wordplay.
'starts to' suggests taking the first letters.
The initial letters of 'believe liberals acting honourably' is 'BLAH'.
(Other definitions for blah that I've seen before include "Bombast" , "Stupid 20" , "Meaningless substitute for tedious words" , "Pretentious words" , "bunkum!" .)