Colonist's dog ate the last lentil (7)
I believe the answer is:
settler
'colonist's' is the definition.
(synonyms)
'dog ate the last lentil' is the wordplay.
'dog' becomes 'setter' (setter is a type of dog).
'ate' is an insertion indicator.
'the last lentil' becomes 'l' (I can't justify this - if you can you should believe this answer much more).
'setter' enclosing 'l' is 'SETTLER'.
Can you help me to learn more?
(Other definitions for settler that I've seen before include "New colonist" , "One colonising new area" , "newcomer, perhaps" , "Colonist, pioneer" , "Coloniser - arbitrator" .)