It's clear that twin girls have a numerous following (5)
I believe the answer is:
vivid
'it's clear' is the definition.
(similar in meaning)
'twin girls have a numerous following' is the wordplay.
'twin girls' can mean two replacements for 'girl'.
'girl' becomes 'vi' (short for Violet).
'girl' becomes 'vi' (short for Violet).
'have' says to put letters next to each other.
'a numerous following' becomes 'd' (I can't justify this - if you can you should give a lot more credence to this answer).
'vi'+'vi'+'d'='VIVID'
'that' acts as a link.
Can you help me to learn more?
(Other definitions for vivid that I've seen before include "Intensely deep/bright" , "Strong and bright, colour perhaps" , "Lifelike, striking" , "Very bright and colourful" , "Brilliantly coloured - full of life" .)