It has a small part in a big picture (5)
I believe the answer is:
inset
'picture' is the definition.
(I know that inset is a type of picture)
'it has a small part in a big' is the wordplay.
'has' means one lot of letters goes inside another.
'a small' becomes 's' (abbreviation - e.g. clothes size).
'part in' is an insertion indicator (inserted letters form part of the word).
'a big' becomes 'ne' (I am not sure about this - if you are sure you should believe this answer much more).
's' put within 'ne' is 'nse'.
'it' enclosing 'nse' is 'INSET'.
Can you help me to learn more?
(Other definitions for inset that I've seen before include "Minor figure" , "Extra page or piece added to enlarge" , "Magazine feature" , "Piece sewn into a garment" , "Synagogue singer" .)