Plain secretaries a politician’s laid into (6)
I believe the answer is:
'plain' is the definition.
(I've seen this before)
'secretaries a politician's laid into' is the wordplay.
'secretaries' becomes 'PAs' (personal assistants).
'politician' becomes 'MP' (Member of Parliament).
'laid into' indicates putting letters inside.
'pas' placed around 'amp' is 'PAMPAS'.
(Other definitions for pampas that I've seen before include "Vast plains of South America" , "7" , "Plains with grass" , "Grassy plains of Argentine" , "South American plain" .)