Political campaign in March, terribly nasty (5,9)
I believe the answer is:
charm offensive
'political campaign' is the definition.
'charm offensive' can be an answer for 'campaign' (I have seen 'smarmy campaign' mean 'charm offensive' so perhaps 'campaign' could also mean 'charm offensive'). I am not certain of the 'political' bit.
'march terribly nasty' is the wordplay.
I cannot really understand how this works, but
an anagram of 'march' is 'charm' which is present in the answer.
The remaining letters 'offensive' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'in' acts as a link.
Can you help me to learn more?
(Other definitions for charm offensive that I've seen before include "Ingratiating campaign is needed" , "Campaign to gain support and become more popular" , "PR campaign needed" , "smarmy campaign" .)