Move of surplus population proves disastrously wrong (9)
I believe the answer is:
overspill
'move of surplus population' is the definition.
'overspill' can be an answer for 'population' (overspill is a kind of population). I'm not certain of the 'move of surplus' bit.
'proves disastrously wrong' is the wordplay.
'disastrously' is an anagram indicator (I've seen this in other clues).
'wrong' becomes 'ill' (I've seen this in another clue).
'proves' is an anagram of 'oversp'.
'oversp'+'ill'='OVERSPILL'
Can you help me to learn more?
(Other definitions for overspill that I've seen before include "Excess; surplus (population)" , "Excess liquid" , "Surplus population moving elsewhere" , "excessive amount" , "for people moving out of slums?" .)