Cooling system fitted in a vehicle for summer, once (6)
I believe the answer is:
abacus
'summer once' is the definition.
'abacus' can be an answer for 'summer' (I've seen this before). I am unsure of the 'once' bit.
'cooling system fitted in a vehicle' is the wordplay.
'cooling system' becomes 'ac' (abbreviation for air conditioner).
'fitted in' indicates putting letters inside.
'vehicle' becomes 'bus' (bus is a kind of vehicle).
'a'+'bus'='abus'
'ac' going within 'abus' is 'ABACUS'.
'for' acts as a link.
Can you help me to learn more?
(Other definitions for abacus that I've seen before include "It might work out" , "Old calculator" , "Very early calculator" , "It might calculate" , "Old counting-frame" .)