The writer’s father’s books about technique in art (7)
I believe the answer is:
'technique in art' is the definition.
(style of painting)
'the writer's father's books about' is the wordplay.
'the writer's' becomes 'I'm' (both could mean 'the crossword setter is').
'father's' becomes 'pa's' (pa can mean dad or father).
'books' becomes 'OT' (abbreviation for Old Testament).
'about' says the letters should be written in reverse.
'ot' reversed gives 'to'.
(Other definitions for impasto that I've seen before include "application of paint in this way" , "artistic technique" , "Slapping paint on" , "Technique of laying on paint thickly so that it stands out from a surface" , "Thick-paint technique" .)