The sadness of father Thomas (6)
I believe the answer is:
pathos
'the sadness' is the definition.
(thesaurus)
'of father thomas' is the wordplay.
I cannot really understand how this works, but
'of' could be 'o' and 'o' is present in the answer.
'father' could be 'pa' (pa can mean dad or father) and 'pa' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for pathos that I've seen before include "Quality that arouses pity or sadness" , "Quality evoking pity or sadness" , "Tragic air" , "Quality arousing pity" , "Quality that arouses sympathy" .)