A flat spin? Deadly! (5)
I believe the answer is:
'deadly' is the definition.
(similar in meaning)
'a flat spin?' is the wordplay.
'spin?' indicates anagramming the letters (I've seen 'spin' mean this).
'aflat' with letters rearranged gives 'FATAL'.
(Other definitions for fatal that I've seen before include "making one late" , "Bringing ruin" , "Resulting in disaster or death" , "Causing death, say accident" , "Calamitous" .)