。
Pale ale is a beer with low alcohol content.
Literal
Pale-ale [topic-は] alcohol content [subj-の] few beer [copula-だ].
The embedded relative clause アルコール含有量の少ない uses の in place of が as the subject marker — a standard option inside relative clauses that feels slightly more written/formal than が. 含有量 'content amount' is a sinitic technical compound productive in science and industry (含有 'containing' + 量 'amount'). The whole relative clause modifies ビール.