Spotted in play its effect is disastrous (6)
I believe the answer is:
domino
'spotted in play its' is the definition.
I can't tell whether this defines the answer.
'effect is disastrous' is the wordplay.
'effect' becomes 'do' (synonyms).
'is disastrous' becomes 'mino' (I can't justify this - if you can you should give a lot more credence to this answer).
'do'+'mino'='DOMINO'
Can you help me to learn more?
(Other definitions for domino that I've seen before include "garment" , "Rectangular tile (with an effect?) used in a table game" , "Eye-mask worn at a masquerade" , "Loose cloak; game piece" , "Black and white game piece" .)