Lay off polychromate treatment for a lightly-built person (9)
I believe the answer is:
ectomorph
'a lightly-built person' is the definition.
I can't tell whether this definition defines the answer.
'lay off polychromate treatment' is the wordplay.
'off' is a deletion indicator ('off' can mean to remove).
'treatment' indicates anagramming the letters (letters treated or processed differently).
'polychromate' with 'lay' taken out is 'pochromte'.
'pochromte' with letters rearranged gives 'ECTOMORPH'.
'for' acts as a link.
Can you help me to learn more?
(Other definitions for ectomorph that I've seen before include "Weed" , "especially delicate type" , "Lean-bodied person" , "Lean creature" .)