Scheme to capture Ireland's first aviator (5)
I believe the answer is:
pilot
'aviator' is the definition.
(pilot is a kind of aviator)
'scheme to capture ireland's first' is the wordplay.
'scheme' becomes 'plot' (plot is a kind of scheme).
'to capture' means one lot of letters goes inside another.
'ireland's first' becomes 'i' (1st letter of 'ireland').
'plot' enclosing 'i' is 'PILOT'.
(Other definitions for pilot that I've seen before include "He steers ship or plane" , "Trial broadcast" , "One who brings a vessel into port" , "who shows the way?" , "Guide, flyer" .)