Money firm when invested in central heating (4,4)
I believe the answer is:
hard cash
'money' is the definition.
(I've seen this before)
'firm when invested in central heating' is the wordplay.
'firm' becomes 'hard' ('hard' can be similar in meaning to 'firm').
'when' becomes 'as'.
'invested in' indicates putting letters inside.
'central heating' becomes 'ch'.
'as' placed into 'ch' is 'cash'.
'hard'+'cash'='HARD CASH'
(Other definitions for hard cash that I've seen before include "Ready money" , "Coins and banknotes - not a cheque" , "Not plastic!" , "Coins and notes, not cheques or promises" .)