Retain; tower (4)
I believe the answer is:
keep
I believe this is a double definition.
'retain' is the first definition.
(I know that retain can be written as keep)
'tower' is the second definition.
(I've seen this before)
(Other definitions for keep that I've seen before include "Conserve" , "Guard" , "Retain; part of castle" , "Save" , "Only one cricketer on field will" .)