Goldfish get aged (3)
I believe the answer is:
old
'aged' is the definition.
(similar in meaning)
'goldfish get' is the wordplay.
'get' indicates a hidden word (I've seen 'got' mean this).
'OLD' can be found hidden inside 'goldfish'.
(Other definitions for old that I've seen before include "Former, previous" , "out of the ark" , "No longer young" , "At an early stage - at a late stage" , "Stale" .)