Peter out in Ilkley, maybe more miserable (7)
I believe the answer is:
moodier
'more miserable' is the definition.
'peter out in ilkley maybe' is the wordplay.
'peter out' becomes 'die'.
'in' means one lot of letters goes inside another.
'ilkley maybe' becomes 'moor' (Ilkley Moor is an example).
'die' going into 'moor' is 'MOODIER'.
(Other definitions for moodier that I've seen before include "more gloomy" , "More sullen" .)