What reduces flow in a petrol station, mostly a resistance (4-4)
I believe the answer is:
agar-agar
'what reduces flow' is the definition.
(thickening agent)
'a petrol station mostly a resistance' is the wordplay.
'petrol station' becomes 'garage' (as in a petrol garage).
'mostly' means to remove the last letter.
'resistance' becomes 'r' (electronics abbreviation).
'garage' with its last letter removed is 'garag'.
'a'+'garag'+'a'+'r'='AGAR-AGAR'
'in' acts as a link.
(Other definitions for agar-agar that I've seen before include "Jelly from seaweed" , "Food thickener from seaweed" , "Thickener obtained from seaweed" , "Gelatinous thickening for food" .)