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' means two substitutions 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 am not sure about this - if you are sure you should believe this answer much more).
'vi'+'vi'+'d'='VIVID'
'that' is the 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" .)