Road to the north pilot marked out in thick liquid (10,4)
I believe the answer is:
evaporated milk
'thick liquid' is the definition.
'evaporated milk' can be an answer for 'liquid' (evaporated milk is a kind of liquid). I am not certain of the 'thick' bit.
'road to the north pilot marked out' is the wordplay.
'road' becomes 'ave' (abbreviation for avenue).
'to the north' is a reversal indicator (in a down clue, letters go up or 'northward').
'out' indicates an anagram (out can mean wrong or inaccurate).
'ave' reversed gives 'eva'.
'pilot'+'marked'='pilotmarked'
'pilotmarked' is an anagram of 'poratedmilk'.
'eva'+'poratedmilk'='EVAPORATED MILK'
'in' acts as a link.
Can you help me to learn more?
(Another definition for evaporated milk that I've seen is " food".)