Invader pilot destroyed? That's fortunate (12)
I believe the answer is:
providential
'fortunate' is the definition.
('providential' can be similar in meaning to 'fortunate')
'invader pilot destroyed?' is the wordplay.
'destroyed?' indicates anagramming the letters.
'invader'+'pilot'='invaderpilot'
'invaderpilot' anagrammed gives 'PROVIDENTIAL'.
'that's' acts as a link.
(Other definitions for providential that I've seen before include "Lucky" , "Unexpectedly fortunate (as if arranged by God)" , "Very opportune" .)