Inland Revenue available to provide firm control (4,4)
I believe the answer is:
iron hand
'firm control' is the definition.
'iron hand' can be an answer for 'control' (I have seen 'Tyrannical control' mean 'iron hand' so perhaps 'control' could also mean 'iron hand'). I am not certain of the 'firm' bit.
'inland revenue available' is the wordplay.
'inland revenue' becomes 'ir' (abbreviation for Inland Revenue).
'available' becomes 'on hand' (similar in meaning).
'ir'+'onhand'='IRON HAND'
'to provide' acts as a link.
Can you help me to learn more?
(Other definitions for iron hand that I've seen before include "inflexible approach" , "It may be found in a velvet glove" , "Proverbially harsh control" , "Firm discipline" , "Strict control" .)