Dam, some say, with northward current -- it's monstrous! (8)
I believe the answer is:
werewolf
'it's monstrous' is the definition.
I can't judge whether this definition defines the answer.
'dam some say with northward current' is the wordplay.
'dam' becomes 'weir' (a weir is a type of dam).
'some say' indicates a 'sounds like' (homophone) clue.
'with' says to put letters next to each other.
'northward' says the letters should be written backwards (in down clue going up).
'current' becomes 'flow' (synonyms).
'weir' sounds like 'were'.
'flow' in reverse letter order is 'wolf'.
'were'+'wolf'='WEREWOLF'
Can you help me to learn more?
(Other definitions for werewolf that I've seen before include "Person beleived to be able to assume lupine form" , "Full moon shape-changer" , "Animal chap?" , "Hairy beast" , "one who can become beastly" .)