Castaway birds on change of course (8,6)
I believe the answer is:
robinson crusoe
'castaway' is the definition.
(I have seen 'Fictional castaway ' mean 'robinson crusoe' so perhaps 'castaway' could also mean 'robinson crusoe')
'birds on change of course' is the wordplay.
'birds' becomes 'robins' (robin is a kind of bird).
'change of' indicates an anagram (the letter order is changed).
'course' with letters rearranged gives 'crusoe'.
'robins'+'on'+'crusoe'='ROBINSON CRUSOE'
(Other definitions for robinson crusoe that I've seen before include "CA [CHILDRENS ADVENTURE]" , "Castaway on desert island" , "shipwrecked hero" , "Novel castaway" , "Defoe's castaway" .)