Airline service cross about supposedly unmissable target? (4,4)
I believe the answer is:
barn door
'supposedly unmissable target?' is the definition.
('broad as a barn door')
'airline service cross about' is the wordplay.
'airline' becomes 'BA' (British Airways).
'service' becomes 'RN' (abbreviation for Royal Navy).
'cross' becomes 'rood' (term for a crucifix).
'about' is a reversal indicator.
'rood' in reverse letter order is 'door'.
'ba'+'rn'+'door'='BARN-DOOR'
(Other definitions for barn door that I've seen before include "What a bad shot can't hit" , "you can't miss it" , "No point in locking it after the horse has bolted" , "Easy target" , "Unmissable target" .)