Isle of Man swimmer's collecting organic fuel (7)
I believe the answer is:
biomass
'organic fuel' is the definition.
'isle of man swimmer's collecting' is the wordplay.
'isle of man' becomes 'iom'.
'swimmer' becomes 'bass' (bass is a kind of fish).
'collecting' indicates putting letters inside.
'iom' going into 'bass' is 'BIOMASS'.
(Other definitions for biomass that I've seen before include "Measure of living organisms" , "source of 8 [RENEWABLE ENERGY]" , "Organic source of energy" .)