Applies to what was formerly in reserve (7)
I believe the answer is:
relates
'applies to what' is the definition.
I can't tell whether this definition defines the answer.
'formerly in reserve' is the wordplay.
'formerly' becomes 'late' (I've seen this in other clues).
'in' means one lot of letters goes inside another.
'reserve' becomes 'res' (abbreviation for reserve).
'late' placed within 'res' is 'RELATES'.
'was' is the link.
Can you help me to learn more?
(Other definitions for relates that I've seen before include "Tells (a tale)" , "Describes" , "couples" , "Narrates" .)