Hard to penetrate word for unit of heat (5)
I believe the answer is:
therm
'unit of heat' is the definition.
(I've seen this before)
'hard to penetrate word' is the wordplay.
'hard' becomes 'h'.
'to penetrate' means one lot of letters goes inside another.
'word' becomes 'term' (term is a kind of word**).
'h' placed into 'term' is 'THERM'.
'for' acts as a link.
(Other definitions for therm that I've seen before include "Amount of heat" , "Unit of heat used in the UK as the basis of gas bills" , "Unit of heat energy" , "it measures heat" , "Gasman's unit" .)