### Clue: Union supporter trips in British Museum – help needed (10)
### Answer: BRIDESMAID 
 ### Explanation: Definition: Union supporter
 RIDES(trips on horseback or in vehicles, say) **contained in**(in) BM(abbrev. for "British Museum") **plus**(… needed) AID(help;assistance).
Defn: Cryptic – A supporting role in a wedding;a matrimonial union.