They say you might need this to start the engine in the shade (5)
I believe the answer is:
khaki
'the shade' is the definition.
(I have seen 'Uniform shade' mean 'khaki' so perhaps 'shade' could also mean 'khaki')
'they say you might need this to start the engine' is the wordplay.
'they say' indicates a 'sounds like' (homophone) clue.
'you might need this to start the engine' becomes 'car key' (ignition).
'carkey' sounds like 'KHAKI'.
'in' is the link.
(Other definitions for khaki that I've seen before include "Military uniform's cloth" , "strong fabric" , "Yellowish brown (used by military)" , "'Colour, often used for military uniforms (5)'" , "Uniform shade" .)