Well-built bouncer finally included in investigation (6)
I believe the answer is:
sturdy
'well-built' is the definition.
(thesaurus)
'bouncer finally included in investigation' is the wordplay.
'bouncer finally' becomes 'r' (I can't explain this - if you can you should believe this answer much more).
'included in' indicates putting letters inside.
'investigation' becomes 'study' (study is a kind of investigation).
'r' inserted into 'study' is 'STURDY'.
Can you help me to learn more?
(Other definitions for sturdy that I've seen before include "Firm" , "Strong and sturdily built" , "Firmly constructed" , "Strong and solidly built" , "Well-constructed" .)