Combine to get retina treated (9)
I believe the answer is:
integrate
'combine' is the definition.
(I know that integrate is a more specific form of the action combine)
'get retina treated' is the wordplay.
'treated' indicates an anagram.
'get'+'retina'='getretina'
'getretina' anagrammed gives 'INTEGRATE'.
'to' is the link.
(Other definitions for integrate that I've seen before include "Incorporate into the whole" , "Organise a get-together" , "Mix" , "Get together" , "Join" .)