Hard time without aid or a bit of heat (5)
I believe the answer is:
therm
'aid or a bit of heat' is the definition.
I can't tell whether this defines the answer.
'hard time without' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'time' becomes 'term' (I've seen this before).
'without' means one lot of letters goes inside another.
'h' placed within 'term' is 'THERM'.
Can you help me to learn more?
(Other definitions for therm that I've seen before include "Unit of heat as in gas supply" , "Unit of heat and gas measure" , "Unit of measurement of heat" , "it measures heat" , "A hot unit" .)