Spenser's enough to keep duke and bequeath permanent income (5)
I believe the answer is:
endow
'bequeath permanent income' is the definition.
(as in leaving an endowment)
'spenser's enough to keep duke' is the wordplay.
'spenser's enough' becomes 'enow' (archaic word for 'enough' in Edmund Spenser's time).
'to keep' means one lot of letters goes inside another.
'duke' becomes 'd' (abbreviation).
'enow' enclosing 'd' is 'ENDOW'.
'and' is the link.
(Other definitions for endow that I've seen before include "supply" , "Enrich (with a facility or gift)" , "Give money to" , "Attribute abilities to" , "provide permanent finance for ..." .)