Doctor astride old gym bike (5)
I believe the answer is:
moped
'bike' is the definition.
(I know that moped is a type of minibike)
'doctor astride old gym' is the wordplay.
'doctor' becomes 'md' (synonyms).
'astride' is an insertion indicator.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'gym' becomes 'pe' (short for Physical Education).
'o'+'pe'='ope'
'md' placed around 'ope' is 'MOPED'.
(Other definitions for moped that I've seen before include "Vehicle - was blue" , "Was in low spirits about the low-powered motorbike" , "dejected" , "Languidly moved" , "Mooned around" .)