Taxi by Welsh lake seen in esoteric lore (7)
I believe the answer is:
cabbala
'esoteric lore' is the definition.
(mystical tradition in Judaism)
'taxi by welsh lake' is the wordplay.
'taxi' becomes 'cab' (synonyms).
'by' is a charade indicator (letters next to each other).
'welsh lake' becomes 'bala' (Lake Bala is a Welsh lake).
'cab'+'bala'='CABBALA'
'seen in' acts as a link.
(Other definitions for cabbala that I've seen before include "special scripture reading" , "Mystical Jewish Bible interpretation" , "Jewish mysticism" , "Jewish mystical tradition" .)