Strange hold-up in river delta (5)
I believe the answer is:
weird
'strange' is the definition.
(synonyms)
'hold-up in river delta' is the wordplay.
'hold-up in river' becomes 'weir' (weir is a type of river dam).
'delta' becomes 'd' (phonetic alphabet: alpha, bravo, charlie etc.).
'weir'+'d'='WEIRD'
(Other definitions for weird that I've seen before include "Creepy" , "Unearthly" , "Uncanny, bizarre" , "Eerie, odd" , "behaving erratically?" .)