Old Royal Marine probing grandma's regional dialect (6)
I believe the answer is:
norman
'regional dialect' is the definition.
(as in Norman French)
'old royal marine probing grandma's' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'royal marine' becomes 'RM' (abbreviation).
'probing' indicates putting letters inside (probe can mean to make a hole in).
'grandma' becomes 'nan' (I've seen this before).
'o'+'rm'='orm'
'orm' placed inside 'nan' is 'NORMAN'.
(Other definitions for norman that I've seen before include "Boy's name" , "Ten sixty six invader" , "A conquest, perhaps" , "he invaded England" , "Someone who's not British" .)