Bill in under a day when shipping escorts (8)
I believe the answer is:
frigates
'shipping escorts' is the definition.
Although both the answer and definition are plural nouns, I cannot see how one could define the other.
'bill in under a day' is the wordplay.
'bill in' becomes 'gates' (Bill Gates. I am not sure about the 'in' bit.).
'under' means one lot of letters go next to another (below, in a down clue).
'a day' becomes 'fri' (abbreviation for Friday).
'gates' after 'fri' is 'FRIGATES'.
'when' is the link.
Can you help me to learn more?
(Other definitions for frigates that I've seen before include "Ships" , "Class of warships" , "boats" .)