What’s hot sun cracking wet earth? (7)
I believe the answer is:
mustard
'what's hot' is the definition.
(I know that mustard is hot)
'sun cracking wet earth?' is the wordplay.
'sun' becomes 'star' (I've seen this in another clue).
'cracking' means one lot of letters goes inside another (inserted letters go into a gap or crack).
'wet earth?' becomes 'mud' (I've seen this in another clue).
'star' put within 'mud' is 'MUSTARD'.
(Other definitions for mustard that I've seen before include "Colonel sometimes suspected" , "Hot-tasting seed" , "Type of Brassica" , "Pungent powder or paste" , "It gives a bit of a kick" .)