Moist pressure beneath reservoir (4)
I believe the answer is:
damp
'moist' is the definition.
(synonyms)
'pressure beneath reservoir' is the wordplay.
'pressure' becomes 'p' (scientific abbreviation).
'beneath' means one lot of letters go next to another (in a down clue, some letters go beneath others).
'reservoir' becomes 'dam' (I can't explain this - if you can you should believe this answer much more).
'p' put after 'dam' is 'DAMP'.
Can you help me to learn more?
(Other definitions for damp that I've seen before include "Moist air" , "Moisture" , "Rising -, TV sitcom" , "dripping" , "(Slightly) wet" .)