Reservoir parking is wet (4)
I believe the answer is:
damp
'is wet' is the definition.
(similar in meaning)
'reservoir parking' is the wordplay.
'reservoir' becomes 'dam' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'parking' becomes 'p' (abbreviation on road signs).
'dam'+'p'='DAMP'
Can you help me to learn more?
(Other definitions for damp that I've seen before include "Rising -, TV sitcom" , "Somewhat moist" , "Moist air" , "(Slightly) wet" , "soggy" .)