Remarkably gruff about pressure mounting (12)
I believe the answer is:
surprisingly
'remarkably' is the definition.
(thesaurus)
'gruff about pressure mounting' is the wordplay.
'gruff' becomes 'surly' (I've seen this before).
'about' indicates putting letters inside.
'pressure' becomes 'p' (abbreviation).
'mounting' becomes 'rising' ('rise' can be a synonym of 'mount'**).
'p'+'rising'='prising'
'surly' going around 'prising' is 'SURPRISINGLY'.
(Other definitions for surprisingly that I've seen before include "Amazingly" , "Unexpectedly" .)