Cultivated, as in large mollusc (5)
I believe the answer is:
snail
'mollusc' is the definition.
(snail is a kind of mollusc)
'cultivated as in large' is the wordplay.
'cultivated' indicates anagramming the letters.
'large' becomes 'L' (eg in clothes sizes).
'as'+'in'='asin'
'asin' is an anagram of 'snai'.
'snai'+'l'='SNAIL'
(Other definitions for snail that I've seen before include "Slow-moving garden pest" , "Slow slithery animal" , "Its speed was emulated by Shakespeare's creeping schoolboy" , "Escargot" , "Soft-bodied garden animal with shell" .)