Alien locked in shower by queen's servant (8)
I believe the answer is:
retainer
'servant' is the definition.
(synonyms)
'alien locked in shower by queen's' is the wordplay.
'alien' becomes 'et' (extra terrestrial).
'locked in' means one lot of letters goes inside another.
'shower' becomes 'rain' (rainfall).
'by' is a charade indicator (letters next to each other).
'queen' becomes 'ER' (short for Elizabeth Regina ie Queen Elizabeth).
'et' put within 'rain' is 'retain'.
'retain'+'er'='RETAINER'
(Other definitions for retainer that I've seen before include "Long-standing family servant" , "Old servant - fee paid in advance" , "The keeper" , "Long-serving member of staff" , "Fee paid to secure services" .)