Absolutely certain company is into table tennis, strangely (13)
I believe the answer is:
incontestable
'absolutely certain' is the definition.
'company is into table tennis strangely' is the wordplay.
'company' becomes 'co' (abbreviation).
'is into' is an insertion indicator.
'strangely' indicates anagramming the letters.
'table'+'tennis'='tabletennis'
'tabletennis' with letters rearranged gives 'inntestable'.
'co' going inside 'inntestable' is 'INCONTESTABLE'.
(Other definitions for incontestable that I've seen before include "sure" , "Without doubt" , "Beyond dispute" .)