Nothing to cover area one’s quick to protect (4)
I believe the answer is:
nail
'one's quick to protect' is the definition.
I can't tell whether this definition defines the answer.
'nothing to cover area' is the wordplay.
'nothing' becomes 'nil' (eg nil is zero in football).
'to cover' is an insertion indicator.
'area' becomes 'a' (maths abbreviation).
'nil' placed around 'a' is 'NAIL'.
Can you help me to learn more?
(Other definitions for nail that I've seen before include "'For want of a . . . . the shoe was lost'" , "Hit this on the head to be absolutely right" , "SECOND PART OF 17" , "Carpentry fastener" , "thing for keeping things together" .)