The sauce of American sailor captured by Civil Defence (7)
I believe the answer is:
custard
'the sauce' is the definition.
(I've seen this before)
'american sailor captured by civil defence' is the wordplay.
'american' becomes 'US' (United States).
'sailor' becomes 'tar' (tar can mean a sailor).
'captured by' means one lot of letters goes inside another.
'civil defence' becomes 'cd'.
'us'+'tar'='ustar'
'ustar' placed inside 'cd' is 'CUSTARD'.
'of' is the link.
(Other definitions for custard that I've seen before include "Yellow mixture" , "Dessert sauce" , "Yellow sauce" , "Pudding accompaniment" , "Type of apple" .)