Base of stem or insect or shag (9)
I believe the answer is:
cormorant
'shag' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'base of stem or insect' is the wordplay.
'base of stem' becomes 'corm' (I can't explain this - if you can you should believe this answer much more).
'insect' becomes 'ant' (ant is a kind of insect).
'corm'+'or'+'ant'='CORMORANT'
'or' is the link.
Can you help me to learn more?
(Other definitions for cormorant that I've seen before include "Large voracious sea bird" , "Black/white fish eater" , "Black diving bird" , "Large, long-necked seabird" , "Aquatic bird" .)