Stupid person needing help to catch flightless bird (7)
I believe the answer is:
airhead
'stupid person' is the definition.
(I've seen this before)
'help to catch flightless bird' is the wordplay.
'help' becomes 'aid' ('aid' can be a synonym of 'help').
'to catch' means one lot of letters goes inside another.
'flightless bird' becomes 'rhea' (rhea is a kind of flightless bird).
'aid' going around 'rhea' is 'AIRHEAD'.
'needing' acts as a link.
(Other definitions for airhead that I've seen before include "Stupid person" , "one with a small brain" , "Fool" , "One's simple" , "Idiot" .)