Rodent, second found in trading place (6)
I believe the answer is:
marmot
'rodent' is the definition.
(I know that marmot is a type of rodent)
'second found in trading place' is the wordplay.
'second' becomes 'mo' (both are short periods of time).
'found in' indicates putting letters inside.
'trading place' becomes 'mart' (I've seen this in another clue).
'mo' put within 'mart' is 'MARMOT'.
(Other definitions for marmot that I've seen before include "Creature" , "Furry burrowing rodent" , "Large burrowing rodent" , "Small burrowing animal of the squirrel family" , "wild rodent" .)