Beast oddly dropped down his horn when retreating (5)
I believe the answer is:
rhino
'beast' is the definition.
(I've seen this before)
'oddly dropped down his horn when retreating' is the wordplay.
'oddly dropped' means one should take alternating letters (letters in odd-numbered positions removed).
'when retreating' is a reversal indicator.
'down'+'his'+'horn'='downhishorn'
The alternating letters of 'downhishorn' are 'onihr'.
'onihr' backwards is 'RHINO'.
(Other definitions for rhino that I've seen before include "Hi, Ron, this animal is abbreviated" , "wild beast" , "one might charge in Africa" , "Powerful herbivorous odd-toed ungulate, informally" , "I may make fearsome charges" .)