When Henry joins Oscar, it's a good sign (4)
I believe the answer is:
halo
'a good sign' is the definition.
(I've seen this in another clue)
'when henry joins oscar' is the wordplay.
I cannot really understand how this works, but
'henry' could be 'hal' (short for Henry) and 'hal' is found in the answer.
'oscar' could be 'o' (phonetic alphabet: alpha, bravo, charlie etc.) and 'o' is found in the answer.
No letters remain.
This explanation may well be incorrect...
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for halo that I've seen before include "Glowing ring (around saint's head)" , "Ring round head of saint" , "Saint's headlight?" , "Circle over a saint" , "Disc of light" .)