Instrument used excessively by a learner? (4)
I believe the answer is:
'instrument' is the definition.
(I know that instrument can be written as tool)
'excessively by a learner?' is the wordplay.
'excessively' becomes 'too' (to too great a degree).
'by' means one lot of letters go next to another.
'a learner?' becomes 'l' (as in a learner driver with L-plates on their car).
'used' acts as a link.
(Other definitions for tool that I've seen before include "Appliance" , "Person used by another" , "Device used for work" , "Drive motor skilfully" , "Cat's-paw; impress design on" .)