Guard largely was familiar with recording commercial (7)

'guard' is the definition.
Both the answer and definition are singular nouns.
Perhaps they are linked in a way I don't understand?

'largely was familiar with recording commercial' is the wordplay.
'largely' means to remove the last letter (most but not all of the letters).
'was familiar with' becomes 'knew' (I've seen this before).
'recording' becomes 'EP' (extended play vinyl record).
'commercial' becomes 'ad' (abbreviation for advertisement).
'knew' with its final letter taken away is 'kne'.
'kne'+'ep'+'ad'='KNEEPAD'

Can you help me to learn more?