Crossword Genius

Fastening for jumper (4)

Ross

I believe the answer is:

frog

I'm a little stuck... Click here to teach me more about this clue!

'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" .)

I've seen this clue in The Guardian.
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