A case for Poirot? That would be appropriate (3)
I believe the answer is:
apt
'appropriate' is the definition.
('apt' can be a synonym of 'appropriate')
'a case for poirot?' is the wordplay.
'case for' suggests removing the centre (the letters which encase the others are used).
'poirot' with its middle taken out is 'pt'.
'a'+'pt'='APT'
'that would be' acts as a link.
(Other definitions for apt that I've seen before include "Tending" , "suitable for the circumstances" , "Liable" , "flat" , "Relevant" .)