Absolutely certain company is into table tennis, strangely (13)
I believe the answer is:
Here is my best explanation:
'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.
'tabletennis' with letters rearranged gives 'inntestable'.
'co' going inside 'inntestable' is 'INCONTESTABLE'.