ἐσθῆτα δὲ φορέουσι οἱ ἱρέες λινέην μούνην why is it fem instead of n pl?
Use your dictionary.
the only thing i have found is λινέη, ἡ, tape-measure used in building. Maybe, stole should be understood?
No. Here’s the sentence again:
ἐσθῆτα δὲ φορέουσι οἱ ἱρέες λινέην μούνην…
Where do you see a neuter plural? There is no neuter plural in the sentence. You may be confusing the ending on ἐσθῆτα with a neuter plural, but it’s actually feminine accusative singular (that’s why Hylander sent you to the dictionary). Secondly, λινέην is the adjective from λινέος, α, ον, contr. -οῦς, ῆ, οῦν, (λίνον) of flax, linen (LSJ). As such, it modifies ἐσθῆτα, and that answers your question.