Flat has covered parking — that's essential (6)
I believe the answer is:
staple
'essential' is the definition.
('staple' can be similar in meaning to 'essential')
'flat has covered parking' is the wordplay.
'flat' becomes 'stale'.
'has covered' is an insertion indicator.
'parking' becomes 'p'.
'stale' placed around 'p' is 'STAPLE'.
'that's' is the link.
(Other definitions for staple that I've seen before include "security device" , "Paper securer; main" , "Main - way to fasten paper" , "Paper pin for plates" , "Fastening wire" .)