Trial of MP involved in deal (6)
I believe the answer is:
sample
'trial' is the definition.
(thesaurus)
'mp involved in deal' is the wordplay.
'involved in' indicates putting letters inside.
'deal' becomes 'sale' (I've seen this in another clue).
'mp' placed into 'sale' is 'SAMPLE'.
'of' acts as a link.
(Other definitions for sample that I've seen before include "instance" , "Small representative piece" , "piece to try" , "Representative portion" , "Take" .)