Need a drink when hot (litres even) during bike races (6)
I believe the answer is:
thirst
'need a drink' is the definition.
'hot litres even during bike races' is the wordplay.
'hot' becomes 'h'.
'even' indicates alternate letters (even-numbered letters are used).
'during' is an insertion indicator.
'bike races' becomes 'tt' (Isle of Man TT motorcycle races).
The alternating letters of 'litres' are 'irs'.
'h'+'irs'='hirs'
'hirs' put into 'tt' is 'THIRST'.
'when' is the link.
(Other definitions for thirst that I've seen before include "The inducement for 5 Down" , "Long" , "Strong desire for drink" , "Need for liquids" , "Quaff to slake it" .)