Small cage holding rodents up, for example (8)
I believe the answer is:
specimen
'example' is the definition.
(I know that specimen is a type of example)
'small cage holding rodents up' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'cage' becomes 'pen' (both can mean an enclosure).
'holding' is an insertion indicator.
'rodents' becomes 'mice' (mouse is a kind of rodent).
'up' is a reversal indicator (in down clue: letters go upwards).
'mice' in reverse letter order is 'ecim'.
'pen' going around 'ecim' is 'pecimen'.
's'+'pecimen'='SPECIMEN'
'for' acts as a link.
(Other definitions for specimen that I've seen before include "Object serving as an example of its class" , "Men's epic (anag.)" , "Test sample" , "blood sample" , "Exemplar" .)