Underground place, mostly nasty and hot on the inside (6)
I believe the answer is:
grotto
'underground place' is the definition.
(type of cave)
'mostly nasty and hot on the inside' is the wordplay.
'mostly' means to remove the last letter.
'nasty' becomes 'grotty' (similar in meaning).
'and' says to put letters next to each other.
'on the inside' means to look at the middle letters.
The centre of 'hot' is 'o'.
'grotty' with its last letter removed is 'grott'.
'grott'+'o'='GROTTO'
(Other definitions for grotto that I've seen before include "Ornamental cave" , "In company (with); over length of" , "A picturesque cave as in Lourdes (6)" , "fancy underground area" , "Fancy cave" .)