Certain moves not nice — steady (13)
I believe the answer is:
incontestable
'certain' is the definition.
(thesaurus)
'moves not nice steady' is the wordplay.
'moves' indicates an anagram.
'steady' becomes 'stable' ('stable' can be similar in meaning to 'steady'**).
'not'+'nice'='notnice'
'notnice' anagrammed gives 'inconte'.
'inconte'+'stable'='INCONTESTABLE'
(Other definitions for incontestable that I've seen before include "Beyond dispute" , "Without doubt" , "sure" .)