I am on top of former old painting technique (7)
I believe the answer is:
impasto
'painting technique' is the definition.
(I've seen this before)
'i am on top of former old' is the wordplay.
'i am' becomes 'im'.
'on top of' is a charade indicator (letters next to each other).
'former' becomes 'past' ('past' can be similar in meaning to 'former').
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'im'+'past'+'o'='IMPASTO'
(Other definitions for impasto that I've seen before include "Style of painting" , "Technique possibly Van Gogh's" , "Technique using thick paint" , "Thick painting technique" , "artistic technique" .)