Father Brown entertains sage, possibly (4)
I believe the answer is:
herb
'sage possibly' is the definition.
(sage is a kind of herb)
'father brown entertains' is the wordplay.
'entertains' indicates the answer is hidden within the clue (the answer is welcomed or entertained by the clue).
'HERB' is hidden in the letters of 'father brown'.
(Other definitions for herb that I've seen before include "Eg, sage" , "Useful or edible plant" , "Simple" , "Medicinal or culinary plant" , "ingredient for recipe?" .)