Catches on to thematic element (5)
I believe the answer is:
twigs
'catches on' is the definition.
('twig' can be a synonym of 'catch on')
'thematic element' is the wordplay.
I cannot quite see how this works, but
'element' could be 'w' (W is an example) and 'w' is present in the answer.
The remaining letters 'tigs' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'to' acts as a link.
Can you help me to learn more?
(Other definitions for twigs that I've seen before include "Slender (tree) shoots" , "Catches on" , "Kindling" , "Slender woody shoots" , "Small branches" .)