Was keen to change small imperfection (8)
I believe the answer is:
weakness
'imperfection' is the definition.
(weakness is a kind of imperfection)
'was keen to change small' is the wordplay.
'to change' is an anagram indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'was'+'keen'='waskeen'
'waskeen' anagrammed gives 'weaknes'.
'weaknes'+'s'='WEAKNESS'
(Other definitions for weakness that I've seen before include "Fault or flaw" , "Failing or shortcoming" , "Defect" , "Ill-defended spot - predilection" , "Vulnerability" .)