。
It really was back-breaking work.
Literal
Truly bone [subject-の] breaks work [was-だった].
Built on the idiom 骨が折れる (lit. 'bones break'), a fixed expression for 'laborious / painstaking work.' Inside the adnominal (relative) clause 骨の折れる, the normal subject marker が is replaced with の — a hallmark of noun-modifying clauses in Japanese, where either particle is grammatical. The whole clause modifies 仕事 ('work'). Subject of the main clause is omitted.