Mostly unpleasant round small cave (6)
I believe the answer is:
grotto
'small cave' is the definition.
(I've seen this before)
'mostly unpleasant round' is the wordplay.
'mostly' means to remove the last letter.
'unpleasant' becomes 'grotty' (synonyms).
'round' becomes 'o' (round can mean a round object).
'grotty' with its last letter taken away is 'grott'.
'grott'+'o'='GROTTO'
(Other definitions for grotto that I've seen before include "Santa's cave" , "fancy underground area" , "(Imitation) cave" , "Ornamental cave" , "Pretty cave" .)