Small bill is miserable (3)
I believe the answer is:
sad
'miserable' is the definition.
('sad' can be similar in meaning to 'miserable')
'small bill' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'bill' becomes 'ad' (advertisement stuck on wall**).
's'+'ad'='SAD'
'is' acts as a link.
(Other definitions for sad that I've seen before include "Melancholy or pitiful" , "Winter blues, initially" , "Heavy-hearted" , "grey" , "Unfortunate" .)