Mostly wonderful, tense batter's innings (6)
I believe the answer is:
wicket
'batter's innings' is the definition.
(cricket)
'mostly wonderful tense' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'wonderful' becomes 'wicked' (slang).
'tense' becomes 't' (grammatical abbreviation).
'wicked' with its last letter taken off is 'wicke'.
'wicke'+'t'='WICKET'
(Other definitions for wicket that I've seen before include "catch, perhaps" , "One of the 21 acrosses" , "Three stumps plus a crosspiece" , "that first-class delivery may take" , "Three stumps and two bails" .)