Saint guided Bob? (4)
I believe the answer is:
sled
'bob?' is the definition.
(I know that bob is a more specific form of sled)
'saint guided' is the wordplay.
'saint' becomes 's'.
'guided' becomes 'led' ('lead' can be a synonym of 'guide').
's'+'led'='SLED'
(Other definitions for sled that I've seen before include "Tray on which to slide" , "Winter transport" , "Snow transport" , "Sledge" , "Vehicle on runners" .)