Number expected to seize large lump (6)
I believe the answer is:
nodule
'lump' is the definition.
(I have seen 'Small lump ' mean 'nodule' so perhaps 'lump' could also mean 'nodule')
'number expected to seize large' is the wordplay.
'number' becomes 'no' (abbreviation for 'number').
'expected' becomes 'due' (I've seen this before).
'to seize' is an insertion indicator.
'large' becomes 'l'.
'due' placed around 'l' is 'dule'.
'no'+'dule'='NODULE'
(Other definitions for nodule that I've seen before include "A small rounded lump" , "Small wartlike lump" , "Small knot" , "Little round lump" , "Small lump or swelling" .)