Poem in book with title you'll be upset about (6,3,4)
I believe the answer is:
little boy blue
'poem' is the definition.
(I've seen this before)
'book with title you'll be upset about' is the wordplay.
'book' becomes 'b'.
'with' means one lot of letters go next to another.
'upset' is an anagram indicator.
'about' indicates putting letters inside.
'title' next to 'youll' is 'titleyoull'.
'titleyoull'+'be'='titleyoullbe'
'titleyoullbe' is an anagram of 'littleoyblue'.
'b' placed inside 'littleoyblue' is 'LITTLE BOY BLUE'.
'in' is the link.
(Other definitions for little boy blue that I've seen before include "Negligent young shepherd" , "Nursery rhyme character - Bill let you bet (anag)" , "Hornblower" , "Nursery rhyme horn blower" , "One exhorted to blow" .)