You and I must leave Tower Hill (3)
I believe the answer is:
tor
'hill' is the definition.
(I know that tor is a type of hill)
'you and i must leave tower' is the wordplay.
'you and i' becomes 'we' (we means you and I).
'must leave' indicates named letters should be taken away.
'tower' with 'we' taken out is 'TOR'.
(Other definitions for tor that I've seen before include "feature of Glastonbury" , "It rises" , "Rot on a hill in Devon" , "eminence" , "Dartmoor peak" .)