Before exercise, leave naval base site (7)
I believe the answer is:
gosport
'naval base site' is the definition.
The definition suggests a singular noun which matches the answer.
'before exercise leave' is the wordplay.
'before' means one lot of letters go next to another.
'exercise' becomes 'sport' (I've seen this before).
'leave' becomes 'go' (to go is to depart or leave).
'sport' put after 'go' is 'GOSPORT'.
Can you help me to learn more?
(Other definitions for gosport that I've seen before include "Facing Pompey" , "Hants town" , "Hampshire coastal town" .)