Spoil one's partner (6)

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

Charade of I’M + PAIR (one’s, or one is, partner). It took me a while to realise that this works only if “partner” and “pair” are construed not as nouns (as the surface grammar would seem to insist) but as verbs, as in “partner/pair up” with someone. Very cunning