。
She's beavering away at it right now.
Literal
She [topic-は] now diligently is-doing.
せっせと is a 擬態語 (state mimetic) capturing diligent, repetitive, almost compulsive activity — head-down work without rest. It typically pairs with verbs of repeated action: せっせと働く ('toil away'), せっせと書く ('write industriously'), せっせと運ぶ ('haul busily'). The bare やる here gets its specifics from context — whatever 'it' is, she's at it tirelessly. Mimetics like this give Japanese a precision-of-manner that English usually has to handle with adverbs or paraphrase.