Joiner entering San Diego (3)
I believe the answer is:
and
'joiner' is the definition.
(I've seen this before)
'entering san diego' is the wordplay.
'entering' indicates the answer is hidden within the clue.
'AND' is hidden in the letters of 'san diego'.
(Other definitions for and that I've seen before include "In addition" , "moreover" , "Together with" , "it needs wholly positive input for positive result" , "It connects words, clauses or sentences" .)