Stone and flower by Welsh lake, heading north (9)
I believe the answer is:
alabaster
'stone' is the definition.
(I've seen this before)
'flower by welsh lake heading north' is the wordplay.
'flower' becomes 'aster' (type of flower).
'by' says to put letters next to each other.
'welsh lake' becomes 'Bala' (Lake Bala is a Welsh lake).
'heading north' says the letters should be written in reverse (in a down clue, letters travel north).
'bala' reversed gives 'alab'.
'aster' after 'alab' is 'ALABASTER'.
'and' acts as a link.
(Other definitions for alabaster that I've seen before include "Sort of gypsum" , "statue material" , "White ornamental stone" , "White substance" , "Form of gypsum used for carving" .)