Invasive presence in part of the bowel is extremely eager (9)
I believe the answer is:
coloniser
'invasive presence in' is the definition.
The definition suggests a singular noun which matches the answer.
'the bowel is extremely eager' is the wordplay.
'the bowel' becomes 'colon' (colon is a kind of bowel).
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'eager' with its middle taken out is 'er'.
'colon'+'is'+'er'='COLONISER'
'part of' is the link.
Can you help me to learn more?
(Other definitions for coloniser that I've seen before include "One's part of the settled establishment" , "Settler in new area" .)