Animal with horn, I suspect (5)
I believe the answer is:
rhino
'animal' is the definition.
(short for rhinoceros)
'with horn i suspect' is the wordplay.
'with' is a charade indicator (letters next to each other).
'suspect' indicates anagramming the letters (a suspicious or unusual spelling).
'horn' put next to 'i' is 'horni'.
'horni' anagrammed gives 'RHINO'.
(Other definitions for rhino that I've seen before include "money in the past" , "Large animal with a horn on its snout, in short" , "Large horned mammal" , "Powerful herbivorous odd-toed ungulate, informally" , "In brief, large thick-skinned animal of Africa" .)