To be seen containing fat (5)
I believe the answer is:
obese
'fat' is the definition.
(similar in meaning)
'to be seen containing' is the wordplay.
'containing' indicates a hidden word.
'OBESE' is hidden within 'to be seen'.
(Other definitions for obese that I've seen before include "Plump" , "Having excessive girth" , "Very big" , "carrying remarkable weight" , "Carrying far too much weight" .)