He proposes a thousand too many (5)
I believe the answer is:
mover
'he proposes a' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'thousand too many' is the wordplay.
'thousand' becomes 'm' (Roman numeral for a thousand).
'too many' becomes 'over' (I've seen this in other clues).
'm'+'over'='MOVER'
Can you help me to learn more?
(Other definitions for mover that I've seen before include ". . . . . and shaker, he or she wields power and influence" , "Someone who proposes a motion for debate" , "Formal proposer" , "One who is in motion - may also be a shaker" , "Dynamic type" .)