Serious pledge (7)
I believe the answer is:
earnest
I believe this is a double definition.
'serious' is the first definition.
(similar in meaning)
'pledge' is the second definition.
(I've seen this before)
(Other definitions for earnest that I've seen before include "Eastern (anag.)" , "Showing sincere and intense conviction" , "guarantee" , "Oscar Wilde play 'The Importance of Being . . . . . . .'" , "Grave" .)