At first had always imagined kind uncle composed poem (5)
I believe the answer is:
haiku
'composed poem' is the definition.
'haiku' can be an answer for 'poem' (haiku is a kind of poem). I am not sure about the 'composed' bit.
'at first had always imagined kind uncle' is the wordplay.
'at first' indicates taking the first letters.
The initial letters of 'had always imagined kind uncle' is 'HAIKU'.
Can you help me to learn more?
(Other definitions for haiku that I've seen before include "Japanese poem of seventeen syllables comprising three lines" , "Japanese 3-line poem of 17 syllables" , "Type of Japanese poem" , "Stylised Japanese poem of 17 syllables" , "Jap 17 syllable poem" .)