Observer - small one with a wheel (7)
I believe the answer is:
spotter
'observer' is the definition.
(I know that spotter is a type of observer)
'small one with a wheel' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'one with a wheel' becomes 'potter' (I've seen this in another clue).
's'+'potter'='SPOTTER'
(Other definitions for spotter that I've seen before include "artist such as Seurat?" , "Observer" , "scout?" , "Spy plane" , "One who looks to train" .)