。
She answered with barely a smile.
Literal
[topic-は] properly smile [object-を] without-showing answered.
ろくに~ない is a fixed pattern meaning 'barely, hardly, not properly' — with ろくに as the adverb and the negation falling on the main verb. 微笑みを見せる ('show a smile') is a common collocation; 見せず is the classical/literary negative ず-form of 見せる, equivalent to modern 見せないで but slightly more terse and formal in flavor. The whole sentence sketches a curt, unwilling reply — the barely-smile is the giveaway that her answer wasn't friendly.