Stand-in monarch's grin — pretence, somehow (6,6)
I believe the answer is:
prince regent
'stand-in monarch's' is the definition.
(prince ruling on behalf of a king or queen)
'grin pretence somehow' is the wordplay.
'somehow' is an anagram indicator.
'grin'+'pretence'='grinpretence'
'grinpretence' anagrammed gives 'PRINCE REGENT'.
(Other definitions for prince regent that I've seen before include "As George IV was during eighteen eleven - eighteen twenty" , "George IV, before he became King" , "George III's son was one such" , "The future George IV in 1811-20" .)