A flat spin? Deadly! (5)
I believe the answer is:
fatal
'deadly' is the definition.
(similar in meaning)
'a flat spin?' is the wordplay.
'spin?' indicates anagramming the letters (I've seen 'spin' mean this).
'a'+'flat'='aflat'
'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" .)