Number turning sour and heavy (7)
I believe the answer is:
onerous
'heavy' is the definition.
(similar in meaning)
'number turning sour' is the wordplay.
'number' becomes 'one' (one is a kind of whole number).
'turning' is an anagram indicator.
'sour' anagrammed gives 'rous'.
'one'+'rous'='ONEROUS'
'and' is the link.
(Other definitions for onerous that I've seen before include "Burdensome, heavy kind of duty" , "weight" , "Heavy, difficult to do or bear" , "Difficult, troublesome" , "Challenging" .)