Terms of reference sometimes limited by vehicles in bill (6)
I believe the answer is:
caress
'bill' is the definition.
(bill can mean to caress or stroke)
'terms of reference sometimes limited by vehicles' is the wordplay.
'terms of' says to take the final letters (term can mean the end of something).
'limited by' means one lot of letters goes inside another (limit can mean to contain or border off).
'vehicles' becomes 'cars' (car is a kind of vehicle).
The final letters of 'reference sometimes' are 'es'.
'es' placed into 'cars' is 'CARESS'.
'in' acts as a link.
(Other definitions for caress that I've seen before include "Stoke affectionately" , "Stroke gently" , "Stroke affectionately" , "Affectionate stroke" , "Tenderly stroke" .)