Diminished worth of pamphlet during performance (9)
I believe the answer is:
detracted
'diminished worth' is the definition.
(I've seen this before)
'pamphlet during performance' is the wordplay.
'pamphlet' becomes 'tract' ('tract' can be a synonym of 'pamphlet').
'during' is an insertion indicator.
'performance' becomes 'deed' (I've seen this in other clues).
'tract' put inside 'deed' is 'DETRACTED'.
'of' is the link.
(Other definitions for detracted that I've seen before include "Took away" , "taken away" , "Diminished worth" .)