Lightweight Dicky trapped in MiG strike (9)
I believe the answer is:
milligram
'lightweight' is the definition.
(I've seen this before)
'dicky trapped in mig strike' is the wordplay.
'dicky' becomes 'ill' (I've seen this before).
'trapped in' means one lot of letters goes inside another.
'strike' becomes 'ram' (I've seen this before).
'ill' placed within 'mig' is 'millig'.
'millig'+'ram'='MILLIGRAM'
(Other definitions for milligram that I've seen before include "not much weight" , "One thousandth of a gram" , "small amount" , "Lightweight" , "Small weight" .)