8A The first few can become harmful (7)
NOISOME
Harmful = def, I see that SOME = few, but I'm not sure how "The first" fits in.
20A I go round to see mother and little one (5)
IMAGO
I can't see a definition in this clue.
5D Familiarise oneself with a charmingly old sound (8)
ACQUAINT
Familiarise oneself = def, charmingly old = QUAINT, but I don't understand "sound" here.
Many thanks as always for assistance.