Enthusiasm for oddly glum sitcom (5)
I believe the answer is:
gusto
'enthusiasm' is the definition.
(gusto is a kind of enthusiasm)
'oddly glum sitcom' is the wordplay.
'oddly' indicates alternate letters (letters in the odd-numbered positions are used).
'glum'+'sitcom'='glumsitcom'
The alternating letters of 'glumsitcom' are 'GUSTO'.
'for' acts as a link.
(Other definitions for gusto that I've seen before include "ardour" , "Great enthusiasm" , "Great relish" , "Got us with great enjoyment" , "Relish; vigour" .)