Mouth-like opening found in enormous tomato (5)
I believe the answer is:
stoma
'mouth-like opening' is the definition.
(stoma is an artificial opening in the body)
'in enormous tomato' is the wordplay.
'in' indicates the answer is hidden within the clue.
'STOMA' is hidden in the letters of 'enormous tomato'.
'found' is the link.
(Other definitions for stoma that I've seen before include "surgical opening" , "Pore in plant" .)