Some Aberdeen terriers join in (5)
I believe the answer is:
enter
'join in' is the definition.
(I've seen this in another clue)
'some aberdeen terriers' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'ENTER' is hidden in the letters of 'aberdeen terriers'.
(Other definitions for enter that I've seen before include "Gain access to" , "Take part in a contest" , "Infiltrate" , "Fruit drink" , "Inscribe" .)