Doctor straddling old exercise bike (5)
I believe the answer is:
moped
'bike' is the definition.
(I know that moped is a type of bike)
'doctor straddling old exercise' is the wordplay.
'doctor' becomes 'md' (synonyms).
'straddling' indicates putting letters inside (straddle can mean to lie on both sides of).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'exercise' becomes 'pe' (Physical Education).
'o'+'pe'='ope'
'md' enclosing 'ope' is 'MOPED'.
(Other definitions for moped that I've seen before include "Light motor cycle" , "Was in low spirits about the low-powered motorbike" , "Motorised bicycle" , "means of transport" , "Mooned around" .)