Warning light - an old airline trick (6)
I believe the answer is:
beacon
'warning light' is the definition.
(I've seen this before)
'an old airline trick' is the wordplay.
'an old airline' becomes 'bea' (British European Airways).
'trick' becomes 'con' (a con is a fraud or trick).
'bea'+'con'='BEACON'
(Other definitions for beacon that I've seen before include "Warning light on a high point" , "Prominent light on a hill" , "warning to others" , "Guilding light" , "Light on a height for navigation or warning" .)