Now and then folded a jumper (4)
I believe the answer is:
flea
'jumper' is the definition.
(something that jumps)
'now and then folded a' is the wordplay.
'now and then' means one should take alternating letters (letters taken on a regular basis, now and then).
The alternating letters of 'folded' are 'fle'.
'fle'+'a'='FLEA'
(Other definitions for flea that I've seen before include "Little jumping insect" , "Jumping, blood-sucking insect" , "Leaping insect" , "Jumping parasite" , "tiny jumper" .)