Gather origin of life must be in fossil fuel (5)
I believe the answer is:
pleat
'gather' is the definition.
(to gather or fold into pleats)
'origin of life must be in fossil fuel' is the wordplay.
'origin of' says to take the initial letters.
'must be in' is an insertion indicator.
'fossil fuel' becomes 'peat' (I can't explain this - if you can you should give a lot more credence to this answer).
The initial letter of 'life' is 'l'.
'l' put within 'peat' is 'PLEAT'.
Can you help me to learn more?
(Other definitions for pleat that I've seen before include "Fold formed in fabric" , "Fold pressed or stitched into cloth" , "Flat fold of cloth" , "Type of fold in fabric" , "Fold sewn or pressed into cloth" .)