Lacking enthusiasm, quietly stops revolutionary eating plan (5)
I believe the answer is:
tepid
'lacking enthusiasm' is the definition.
(I've seen this in another clue)
'quietly stops revolutionary eating plan' is the wordplay.
'quietly' becomes 'p' (musical symbol).
'stops' means one lot of letters goes inside another (stopping can mean blocking a gap).
'revolutionary' says the letters should be written backwards (letters revolve around).
'eating plan' becomes 'diet' (I've seen this in another clue).
'diet' written backwards gives 'teid'.
'p' put within 'teid' is 'TEPID'.
(Other definitions for tepid that I've seen before include "without enthusiasm" , "lacking enthusiasm" , "Not very warm (water)" , "What's a bit warm" , "Half-hearted" .)