Cheers a tallish monk in, wildly (6,1,7)
I believe the answer is:
thanks a million
'cheers' is the definition.
(cheers can mean thank you)
'a tallish monk in wildly' is the wordplay.
'wildly' is an anagram indicator.
'a'+'tallish'+'monk'+'in'='atallishmonkin'
'atallishmonkin' with letters rearranged gives 'THANKS A MILLION'.
(Other definitions for thanks a million that I've seen before include "(Often ironic) expression of gratitude" , "that's very much appreciated! - with 18d" , "that's very much appreciated - with 8a" , "I appreciate that" .)