Add up at the pithead where the oil may be (4)
I believe the answer is:
sump
'where the oil may be' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'add up at the pithead' is the wordplay.
'add up' becomes 'sum' (synonyms).
'at' means one lot of letters go next to another.
'the pithead' becomes 'p' (head letter of 'pit').
'sum'+'p'='SUMP'
Can you help me to learn more?
(Other definitions for sump that I've seen before include "Lubrication reservoir" , "Into which liquids drain" , "Pit receiving drainage" , "Cesspool" , "Engine oil reservoir" .)