Large tin becomes essential (8)
I believe the answer is:
integral
'essential' is the definition.
(I've seen this before)
'large tin becomes' is the wordplay.
'becomes' indicates an anagram.
'large'+'tin'='largetin'
'largetin' with letters rearranged gives 'INTEGRAL'.
(Other definitions for integral that I've seen before include "Belonging as an essential part" , "Forming an essential part" , "form whole" , "Maths function; entire" , "contributing to unity" .)