Fail to survive on an inadequate diet (3)
I believe the answer is:
die
'fail' is the definition.
(synonyms)
'inadequate diet' is the wordplay.
'inadequate' means to remove the last letter (an incomplete or inadequate form of the word).
'diet' with its final letter taken away is 'DIE'.
'to survive on an' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for die that I've seen before include "One spotted" , "Pass away" , "One of a pair of dice" , "Stop functioning" , "Numbered cube" .)