Close relation starts to sing in shower (3)
I believe the answer is:
sis
'close relation' is the definition.
(short for sister)
'starts to sing in shower' is the wordplay.
'starts to' says to take the initial letters.
The first letters of 'sing in shower' is 'SIS'.
(Other definitions for sis that I've seen before include "Secret Intelligence Service (abbr.)" , "Secret Intelligence Service (abbreviation)" , "Little sibling" , "spies ..." , "Girl" .)