An element of pressure over revised shop hours (10)
I believe the answer is:
phosphorus
'an element' is the definition.
(I know that phosphorus is a type of element)
'pressure over revised shop hours' is the wordplay.
'pressure' becomes 'p' (scientific abbreviation).
'over' means one lot of letters go next to another (in a down clue, some letters go over others).
'revised' indicates anagramming the letters.
'shop'+'hours'='shophours'
'shophours' with letters rearranged gives 'hosphorus'.
'p'+'hosphorus'='PHOSPHORUS'
'of' is the link.
(Other definitions for phosphorus that I've seen before include "It lights" , "Element, symbol P" , "Flammable illuminating material" , "Combustible element" , "Element used in making matches" .)