Who sings? Smooth Rat Pack member Sinatra, to start with (4,7)

'sinatra to start with' is the definition.
I can't judge whether this defines the answer.

'who sings? smooth rat pack member' is the wordplay.
I cannot really understand how this works, but
'smooth' could be 'sand' (sanding is a kind of smoothing) and 'sand' is found in the answer.
an anagram of 'rat' is 'art' which is present in the answer.
'member' could be 'm' and 'm' is present in the answer.
The remaining letters 'ins' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?