More than one bulb lit up, then switch is on (6)
I believe the answer is:
onions
'more than one bulb' is the definition.
(onion is a kind of bulb)
'lit up then switch is on' is the wordplay.
'lit up' becomes 'on' (I can't explain this - if you can you should give a lot more credence to this answer).
'then' means one lot of letters go next to another.
'switch' indicates anagramming the letters.
'is'+'on'='ison'
'ison' with letters rearranged gives 'ions'.
'on'+'ions'='ONIONS'
Can you help me to learn more?
(Other definitions for onions that I've seen before include "Experts know theirs" , "If you know your vegetables, you're well up" , "Tearful bulbs" , "England medium-fast bowler" , "Famous lexicographer" .)