Law #1: study vocabulary (7)
I believe the answer is:
lexicon
'vocabulary' is the definition.
(I've seen this before)
'law 1 study' is the wordplay.
'law' becomes 'lex' (Latin for 'law').
'1' becomes 'i' ('i' can be a synonym of '1').
'study' becomes 'con' (I've seen this before).
'lex'+'i'+'con'='LEXICON'
(Other definitions for lexicon that I've seen before include "Vocabulary - dictionary" , "Wordbook" , "Dictionary of terms" , "Dictionary as of Greek or Hebrew" , "reference work?" .)