Burdensome, being single, terribly sour (7)
I believe the answer is:
onerous
'burdensome' is the definition.
(I know that burdensome can be written as onerous)
'single terribly sour' is the wordplay.
'single' becomes 'one' (a single thing - only one).
'terribly' is an anagram indicator.
'sour' with letters rearranged gives 'rous'.
'one'+'rous'='ONEROUS'
'being' is the link.
(Other definitions for onerous that I've seen before include "Burdensome, weighty" , "Heavy, difficult to do or bear" , "Oppressive, difficult to bear" , "weight" , "No euros? That's burdensome" .)