Unpleasant, small, miniscule, bleak ending included (6)
I believe the answer is:
stinky
'unpleasant' is the definition.
(I've seen this in another clue)
'small miniscule bleak ending included' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'miniscule' becomes 'tiny' (I've seen this in another clue).
'ending' indicates one should take the final letters (the ending of the word).
'included' indicates putting letters inside.
The last letter of 'bleak' is 'k'.
'tiny' going around 'k' is 'tinky'.
's'+'tinky'='STINKY'
(Other definitions for stinky that I've seen before include "Foul-smelling" , "Having an unpleasant smell" , "high" , "Odoriferous" , "Humming" .)