Prime Minister retaining large yen to be fit (7)
I believe the answer is:
healthy
'be fit' is the definition.
('healthy' is associated in meaning with 'fit')
'prime minister retaining large yen' is the wordplay.
'prime minister' becomes 'heath' (Edward Heath).
'retaining' is an insertion indicator.
'large' becomes 'l'.
'yen' becomes 'y'.
'heath' placed around 'l' is 'health'.
'health'+'y'='HEALTHY'
'to' is the link.
(Other definitions for healthy that I've seen before include "Hale and hearty" , "In good condition" , "Physically sound" , "Fit, in sound physical shape" , "Fit, undiseased" .)