Under pressure upset son talks foolishly (8)
I believe the answer is:
prattles
'talks foolishly' is the definition.
(I've seen this before)
'under pressure upset son' is the wordplay.
'under' means one lot of letters go next to another (in a down clue, some letters go below others).
'pressure' becomes 'p' (abbreviation).
'upset' becomes 'rattle' (I've seen this in another clue).
'son' becomes 's' (genealogical abbreviation for son).
'p'+'rattle' is 'prattle'.
'prattle'+'s'='PRATTLES'
(Other definitions for prattles that I've seen before include "talks foolishly" , "Speaks incessantly about uninteresting subjects" .)