Phone up after pressure is checked (5)
I believe the answer is:
plaid
'is checked' is the definition.
(plaid is a checked pattern)
'phone up after pressure' is the wordplay.
'phone' becomes 'dial' (dial phone is a kind of phone).
'up' is a reversal indicator (in down clue: letters go upwards).
'after' means one lot of letters go next to another.
'pressure' becomes 'p' (abbreviation).
'dial' written backwards gives 'laid'.
'laid' put after 'p' is 'PLAID'.
(Other definitions for plaid that I've seen before include "Check fabric" , "Tartan-pattern cloth" , "Cloth with tartan pattern" , "Scotsman's cloth" , "Scottish Highland fabric" .)