Memorial is large one occupying field (5)
I believe the answer is:
relic
'memorial' is the definition.
(object kept to remember a holy person)
'large one occupying field' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'one' becomes 'i' (Roman numeral).
'occupying' means one lot of letters goes inside another (inserted letters occupy or enter the word).
'field' becomes 'rec' (short for recreation ground).
'l'+'i'='li'
'li' placed into 'rec' is 'RELIC'.
'is' acts as a link.
(Other definitions for relic that I've seen before include "Personal saintly memorial" , "Corporal remains" , "Survival from earlier times" , "Item surviving from the past" , "Saint's body part?" .)