Substitute also available in fancy tins (5,2)
I believe the answer is:
stand in
'substitute' is the definition.
(I know that stand-in is a type of substitute)
'also available in fancy tins' is the wordplay.
'also' becomes 'and' (both can mean in addition).
'available in' means one lot of letters goes inside another.
'fancy' indicates an anagram.
'tins' is an anagram of 'stin'.
'and' inserted inside 'stin' is 'STAND-IN'.
(Other definitions for stand in that I've seen before include "Double" , "Deputise; sail close to the shore" , "locum" , "It could be night-watchman" , "cover" .)