。
The only sound I could hear was the ticking of a clock.
Literal
Audible sound [topic-は] clock [possessive-の] tick-tick [quotative-という] sound only was.
カチカチ is a 擬音語 (sound mimetic) for sharp, repetitive clicking — here, a clock ticking. ~という marks the preceding word as a description or name: 'the sound called tick-tick.' ~だけ limits the scope to 'only that.' The sentence structure is topic (聞こえる音) + comment (時計のカチカチという音だけだった).