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it looks like a wasp!! ;)
This fly is spiders' parasitoid. Under flies I spotted Thanatus cf. vulgaris/atratus (Philodromidae) , and some Pardosa sp. and Hogna radiata (both Lycosidae) wandering around.
But it seems that acrocerids.. "apparently have no fixed host-parasitoid relationships at the species level". And attacks spiders that make web? (not sure about this. But lycosids and philodromids don?t build web. )