Mucky stuff pleasant? Not entirely (3)
I believe the answer is:
goo
'mucky stuff' is the definition.
(slimy or mucky substance)
'pleasant? not entirely' is the wordplay.
'pleasant?' becomes 'good'.
'not entirely' means to remove the last letter.
'good' with its last letter removed is 'GOO'.
(Other definitions for goo that I've seen before include "Sticky and slimy stuff" , "Sentimentality (sl.)" , "mud, for example" , "Sticky gunge" , "Sticky stuff or sickly sentiment" .)