Unusual collection of shell-like blocks fell backwards (7)
I believe the answer is:
exotica
'unusual collection' is the definition.
(I've seen this in another clue)
'shell-like blocks fell backwards' is the wordplay.
'shell-like' becomes 'otic' (relating to the ears - ears resembles shells).
'blocks' is an insertion indicator (the inside letters form a 'blockage').
'fell' becomes 'axe' (fell can mean to chop down with an axe).
'backwards' says the letters should be written in reverse.
'axe' backwards is 'exa'.
'otic' going within 'exa' is 'EXOTICA'.
'of' acts as a link.
(Other definitions for exotica that I've seen before include "Strange foreign objects" , "Foreign or unfamiliar things" , "Strangely foreign objects" , "Unusual articles" , "Unusual foreign objects" .)