Replacement of trouser damaged round top of groin area (9)
I believe the answer is:
surrogate
'replacement' is the definition.
(thesaurus)
'trouser damaged round top of groin area' is the wordplay.
'damaged' is an anagram indicator.
'round' indicates putting letters inside.
'top of' indicates taking the first letters.
The initial letters of 'groin area' is 'ga'.
'trouser' is an anagram of 'surrote'.
'surrote' going around 'ga' is 'SURROGATE'.
'of' acts as a link.
(Other definitions for surrogate that I've seen before include "(A) substitute in office" , "Deputising substitute" , "Argues about rot with substitute or deputy" , "One standing in" , "Substitute fulfilling the role of another" .)