Day before fast run, tucking into bananas, I cut grass (5,4)
I believe the answer is:
mardi gras
'day' is the definition.
(Mardi Gras is an example)
'run tucking into bananas i cut grass' is the wordplay.
'run' becomes 'r' (cricket abbreviation).
'tucking into' means one lot of letters goes inside another.
'bananas' becomes 'mad' (slang for insane).
'cut grass' becomes 'gras' (I can't explain this - if you can you should believe this answer much more).
'r' placed inside 'mad' is 'mard'.
'mard'+'i'+'gras'='MARDI GRAS'
'before fast' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for mardi gras that I've seen before include "Shrove Tuesday carnival" , "Shrove Tuesday celebration" , "festive occasion" , "Day for 9 acrosses" , "Shrove Tuesday festival" .)