Small pressure against early locomotive wheel (8)
I believe the answer is:
sprocket
'wheel' is the definition.
(sprocket is a kind of wheel)
'small pressure against early locomotive' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'pressure' becomes 'p' (abbreviation).
'against' is a charade indicator (letters next to each other).
'early locomotive' becomes 'rocket' (Stephenson's Rocket, steam locomotive).
's'+'p'+'rocket'='SPROCKET'
(Other definitions for sprocket that I've seen before include "Wheel projection engaging with hole in, eg, film" , "Part of a wheel" , "Roller with teeth" , "sort of wheel" , "Toothed wheel driven by chain" .)