Secure mesh mounted below nocturnal thing (6)
I believe the answer is:
batten
'secure' is the definition.
(synonyms)
'mesh mounted below nocturnal thing' is the wordplay.
'mesh' becomes 'net' (synonyms).
'mounted' says the letters should be written in reverse.
'below' says to put letters next to each other.
'nocturnal thing' becomes 'bat' (I can't explain this - if you can you should give a lot more credence to this answer).
'net' reversed gives 'ten'.
'ten' after 'bat' is 'BATTEN'.
Can you help me to learn more?
(Other definitions for batten that I've seen before include "Fasten securely - say the hatches" , "Wooden stiffening strip" , "Gain nourishment" , "Piece of securing wood" , "Fastener for tarpaulin" .)