Trace missing account on unknown dinosaur (1,3)
I believe the answer is:
t rex
'dinosaur' is the definition.
(abbreviation for Tyrannosaurus Rex)
'trace missing account on unknown' is the wordplay.
'missing' indicates named letters should be taken away.
'account' becomes 'ac' (abbreviation for account).
'on' means one lot of letters go next to another.
'unknown' becomes 'x' (algebra).
'trace' with 'ac' taken out is 'tre'.
'tre'+'x'='T-REX'
(Other definitions for t rex that I've seen before include "1960-70s' glam rock band" , "Marc Bolan's group" , "Long, difficult journey" .)