How do I look? Does it suit me? It's the servant's outfit they wear around here, you see.
Literal
How is-it? Suit-[are-ている]? Around-here [at-で] use servant [gen-の] clothing [explanation-なんですよ].
ここいら is a casual spoken contraction of このあたり ('around here'). 使う modifies 服 as a relative clause — the clothing used around here. 似合う ('to suit, to look good on') takes に for the wearer in full sentences (私に似合う), though it's elided here. んですよ stacks the explanatory の with the assertive よ, 'it's, you see, this...' 使用人 ('servant') is a period/fiction-flavored word; modern households would more likely use お手伝いさん or ハウスキーパー.