Strange dam on Dee (5)
I believe the answer is:
weird
'strange' is the definition.
('weird' can be a synonym of 'strange')
'dam on dee' is the wordplay.
'dam' becomes 'weir' (a weir is a type of dam**).
'on' is a charade indicator (letters next to each other).
'dee' becomes 'd'.
'weir'+'d'='WEIRD'
(Other definitions for weird that I've seen before include "Quirky" , "Unearthly" , "Strange, peculiar" , "Creepy" , "Uncanny, bizarre" .)