Additional fish eater has change of heart (5)
I believe the answer is:
other
'additional' is the definition.
(I've seen this before)
'fish eater has change of heart' is the wordplay.
'fish eater' becomes 'oth' (I can't explain this - if you can you should give a lot more credence to this answer).
'has' says to put letters next to each other.
'change' becomes 'conversion' (conversion is a kind of change).
'of heart' means to look at the middle letters (I've seen 'heart of' mean this).
The middle letters of 'conversion' are 'er'.
'oth'+'er'='OTHER'
Can you help me to learn more?
(Other definitions for other that I've seen before include "Different, disparate" , "Throe (anag)" , "Different (from those mentioned)" , "else!" , "not like you" .)