A change of insides — ref is on top of it (5)
I believe the answer is:
refit
'a change of insides' is the definition.
'ref is on top of it' is the wordplay.
'is on top of' says to put letters next to each other.
'ref'+'it'='REFIT'
(Other definitions for refit that I've seen before include "Repair - renew" , "dockyard job" , "Mend" , "Do repair job" , "Updating or repair process for a ship" .)