The inside alternatively is extra (5)
I believe the answer is:
other
'is extra' is the definition.
(I've seen this in another clue)
'the inside alternatively' is the wordplay.
'inside' indicates putting letters inside.
'alternatively' becomes 'or'.
'the' put within 'or' is 'OTHER'.
(Other definitions for other that I've seen before include "Different, not the same one or this one" , "Not the one mentioned" , "Different; alternate" , "not like you" , "Different, disparate" .)