Cook may need this when sailor's in the West End (6)
I believe the answer is:
wasabi
'cook may need this' is the definition.
The definition suggests a singular noun which matches the answer.
'when sailor's in the west end' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'sailor' becomes 'ab' (abbreviation for able seaman).
'in' means one lot of letters goes inside another.
'the west end' becomes 'wi'.
'as'+'ab'='asab'
'asab' placed within 'wi' is 'WASABI'.
Can you help me to learn more?
(Other definitions for wasabi that I've seen before include "'Japanese plant, often eaten with raw fish (6)'" , "Pungent green root used in Japanese cookery" , "Hollyhock" , "Sushi condiment" , "Strong-tasting Japanese paste" .)