It is unavoidable, distress when given dirt about son (5,4)
I believe the answer is:
needs must
'it is unavoidable' is the definition.
(I've seen this before)
'distress when given dirt about son' is the wordplay.
'distress' becomes 'need' (I've seen this in another clue).
'when given dirt' becomes 'must' (I can't justify this - if you can you should give a lot more credence to this answer).
'about' is a charade indicator (letters next to each other).
'son' becomes 's' (genealogical abbreviation for son).
'must' put after 's' is 'smust'.
'need'+'smust'='NEEDS MUST'
Can you help me to learn more?
(Other definitions for needs must that I've seen before include "Perforce" , "Of necessity" , "'. . . . . . . . . when the devil drives'" , "It is necessary" , "It's inevitable" .)