Grand day to open fifty mayonnaise plants (8)
I believe the answer is:
gladioli
'plants' is the definition.
The answer is an example.
'grand day to open fifty mayonnaise' is the wordplay.
'grand' becomes 'g' (G is short for grand as in a thousand).
'day' becomes 'd' (abbreviation for day).
'to open' indicates putting letters inside (some letters open a gap in others).
'fifty' becomes 'l' (Roman numeral).
'mayonnaise' becomes 'aioli' (mayonnaise seasoned with garlic).
'l'+'aioli'='laioli'
'd' going into 'laioli' is 'ladioli'.
'g'+'ladioli'='GLADIOLI'
(Other definitions for gladioli that I've seen before include "bloomers!" , "Tall garden flowers" , "Garden plants of the iris family" , "Plant with brightly coloured, funnel-shaped flowers" , "Did the Iliad log out these flowers?" .)