Reliable Republican probes American moneylenders (7)
I believe the answer is:
usurers
'moneylenders' is the definition.
('usurer' can be a synonym of 'moneylender')
'reliable republican probes american' is the wordplay.
'reliable' becomes 'sure' ('sure' can be similar in meaning to 'reliable').
'republican' becomes 'r' (abbreviation).
'probes' means one lot of letters goes inside another.
'american' becomes 'us'.
'sure'+'r'='surer'
'surer' placed within 'us' is 'USURERS'.
(Other definitions for usurers that I've seen before include "Loan sharks" , "Biblical bankers who overcharged" , "They lends money at excessive rates of interest" , "They charge excessive interest" .)