Spars with parent boxing in street with son (9)

I haven't cracked this one yet — but the brilliant team at Times for the Times have:

I would have said that spars were the transverse rods that support the sails rather than the masts, which held me up a while, but I’m prepared to accept Chambers’ definition of spar(s) as being “a general term for masts, yards, booms, gaffs, etc”. Anyway, it’s parent MAMA with IN enclosed followed by ST[reet] and S[on]