End of majestic spoonerism ruined concentration (11)
I believe the answer is:
compression
'concentration' is the definition.
(I know that compression is a type of concentration)
'end of majestic spoonerism ruined' is the wordplay.
'end of' indicates one should take the final letters.
'ruined' indicates anagramming the letters.
The final letter of 'majestic' is 'c'.
'spoonerism' with letters rearranged gives 'ompression'.
'c'+'ompression'='COMPRESSION'
(Other definitions for compression that I've seen before include "Forced reduction in volume (of gas)" , "Process of compaction" , "Process of being forcibly condensed" , "Act of forcing together" .)