Strength in art debatable, surely (3,7)
I believe the answer is:
for certain
'surely' is the definition.
('for certain' can be a synonym of 'surely')
'strength in art debatable' is the wordplay.
I cannot quite see how this works, but
'strength' could be 'force' ('force' can be a synonym of 'strength') and 'force' is present in the answer.
'in' is found in the answer.
an anagram of 'art' is 'rta' which is present in the answer.
This accounts for all the letters.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for for certain that I've seen before include "without a doubt" , "surely" , "definitely" , "Assuredly" .)