Tim upset about two fellows causing damage (10)
I believe the answer is:
impairment
'damage' is the definition.
(I know that impairment is a type of damage)
'tim upset about two fellows' is the wordplay.
'upset' is an anagram indicator.
'about' is an insertion indicator.
'two' becomes 'pair' (pair is a kind of two).
'fellows' becomes 'men' (fellow can mean a man).
'tim' anagrammed gives 'imt'.
'pair'+'men'='pairmen'
'imt' placed around 'pairmen' is 'IMPAIRMENT'.
'causing' is the link.
(Other definitions for impairment that I've seen before include "Disability" , "Weakening, damage" , "Injury" .)