Extremely masterly programme featuring tense whodunits (9)
I believe the answer is:
mysteries
'whodunits' is the definition.
('mystery' can be a synonym of 'whodunit')
'extremely masterly programme featuring tense' is the wordplay.
'extremely' means to remove the middle letters.
'programme' becomes 'series' (series is a kind of programme).
'featuring' is an insertion indicator.
'tense' becomes 't' (abbreviation used in many dictionaries).
'masterly' with its centre removed is 'my'.
'series' enclosing 't' is 'steries'.
'my'+'steries'='MYSTERIES'
(Other definitions for mysteries that I've seen before include "Enigmas, puzzles" , "closed books..." , "Inexplicable things" , "in which priests classically played leading role" , "Crime fiction" .)