Intensity of feeling is hard to take in (4)
I believe the answer is:
heat
'intensity of feeling' is the definition.
(I've seen this in another clue)
'hard to take in' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'to take in' becomes 'eat' (to eat is to take in food).
'h'+'eat'='HEAT'
'is' acts as a link.
(Other definitions for heat that I've seen before include "Preliminary round in a contest" , "Energy that caues a rise in temperature" , "Cook" , "early round" , "Warm up" .)