Fellow generated heat lower down (7)
I believe the answer is:
beneath
'lower down' is the definition.
(beneath means below or lower down)
'fellow generated heat' is the wordplay.
'fellow' becomes 'ben' (I can't justify this - if you can you should give a lot more credence to this answer).
'generated' indicates anagramming the letters.
'heat' is an anagram of 'eath'.
'ben'+'eath'='BENEATH'
Can you help me to learn more?
(Other definitions for beneath that I've seen before include "At a lower place" , "At a lower level (than)" , "Inferior to" , "Directly under" , "Unbefitting" .)