Tough king retaining the monarchy in the end (8)
I believe the answer is:
leathery
'tough' is the definition.
('leathery' can be a synonym of 'tough')
'king retaining the monarchy in the end' is the wordplay.
'king' becomes 'lear' (I've seen this before**).
'retaining' indicates putting letters inside.
'in the end' suggests the final letters.
The last letter of 'monarchy' is 'y'.
'lear' placed around 'the' is 'leather'.
'leather'+'y'='LEATHERY'
(Other definitions for leathery that I've seen before include "(Of skin) having a tough and hard texture" , "Tough like tanned hide" , "Tough-textured" , "as old balls?" , "Tough, wrinkled (skin)" .)