Mostly inadequate sign left by an animal (5)
I believe the answer is:
spoor
'sign left by an animal' is the definition.
'spoor' can be an answer for 'sign' (thesaurus). I'm not certain of the 'left by an animal' bit.
'mostly inadequate' is the wordplay.
'mostly' becomes 's' (I am not sure about this - if you are sure you should believe this answer much more).
'inadequate' becomes 'poor' (synonyms).
's'+'poor'='SPOOR'
Can you help me to learn more?
(Other definitions for spoor that I've seen before include "Track of wild animal" , "Trace" , "Animal's track/scent" , "Animal track or scent" , "24 [TRAIL]" .)