A jump start’s a bit doubtful for the moment (5)
I believe the answer is:
jiffy
'the moment' is the definition.
(thesaurus)
'a jump start's a bit doubtful for' is the wordplay.
'start's' says to take the initial letters (I've seen 'start' mean this).
'a bit doubtful for' becomes 'iffy' (I can't explain this - if you can you should believe this answer much more).
The first letter of 'jump' is 'j'.
'j'+'iffy'='JIFFY'
Can you help me to learn more?
(Other definitions for jiffy that I've seen before include "Moment (slang)" , "Two shakes of a lamb's tail?" , "Brief moment" , "Split second" , "Very short moment - type of bag" .)