Spy dog smuggles contents of plan in little bits (9)
I believe the answer is:
molecular
'spy dog smuggles contents of plan in little bits' is the definition.
I can't tell whether this definition defines the answer.
'spy dog smuggles contents of plan' is the wordplay.
'spy' becomes 'mole' (mole is a kind of spy).
'dog' becomes 'cur' (a cur is an aggressive or scruffy dog).
'smuggles' is an insertion indicator.
'contents of' says to take the centre (letters inside the word).
The central letters of 'plan' are 'la'.
'cur' placed around 'la' is 'cular'.
'mole'+'cular'='MOLECULAR'
Can you help me to learn more?
(Other definitions for molecular that I've seen before include "Relating to the smallest particles of a substance" , "Relating to groups of atoms bonded together" , "Dealing with small particles" .)