Fastening for jumper (4)
I believe the answer is:
frog
'jumper' is the definition.
(I've seen this before)
'fastening for' is the wordplay.
I cannot quite see how this works, but
an anagram of 'for' is 'fro' which is within the answer.
A single letter 'g' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for frog that I've seen before include "One of Ranidae" , "Amphibious jumper" , "Long-legged amphibian" , "Adult tadpole" , "throat problem" .)