Make changes to help pair secure operation (3,11)
I believe the answer is:
hip replacement
'operation' is the definition.
(type of surgical operation)
'make changes to help pair secure' is the wordplay.
'make changes to' is an anagram indicator.
'secure' becomes 'cement' (cementing is a kind of securing).
'help'+'pair'='helppair'
'helppair' anagrammed gives 'hiprepla'.
'hiprepla'+'cement'='HIP REPLACEMENT'
(Other definitions for hip replacement that I've seen before include "Inserted joint" , "Joint operation" .)