Blathered on, unnerved by pressure (8)
I believe the answer is:
prattled
'blathered' is the definition.
(I've seen this in another clue)
'unnerved by pressure' is the wordplay.
'unnerved' becomes 'rattled' (I've seen this before).
'by' means one lot of letters go next to another.
'pressure' becomes 'p' (abbreviation).
'rattled' put after 'p' is 'PRATTLED'.
'on' acts as a link.
(Other definitions for prattled that I've seen before include "Chattered" , "Wittered on" .)