Moisturises as result of unusually dry heat and sun, principally (8)
I believe the answer is:
hydrates
'moisturises' is the definition.
(hydrating is a kind of moisturizing)
'unusually dry heat and sun principally' is the wordplay.
'unusually' is an anagram indicator.
'and' says to put letters next to each other.
'principally' indicates taking the first letters.
The first letter of 'sun' is 's'.
'dry'+'heat'='dryheat'
'dryheat' anagrammed gives 'hydrate'.
'hydrate'+'s'='HYDRATES'
'as result of' is the link.
(Another definition for hydrates that I've seen is " Combines with water".)