Wader, on deserting one grazer (4)
I believe the answer is:
ibis
'wader' is the definition.
(I know that ibis is a type of wader)
'deserting one grazer' is the wordplay.
I cannot quite understand how this works, but
'one' could be 'i' (Roman numeral) and 'i' is found within the answer.
The remaining letters 'bis' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'on' is the link.
Can you help me to learn more?
(Other definitions for ibis that I've seen before include "leggy bird" , "Eastern" , "Wader once sacred in Egypt" , "Near-colourless mineral" , "Type of bird" .)