The entire headword phrase can function idiomatically like an adverb meaning 'almost', as at, for example, Demosthenes 27.29 (web address 1). It is composed of mikrou=, genitive singular of mikro/s, ('small'), and dei=n, the present active infinitive of the impersonal verb dei=, ('there is need') or ('it is necessary').
See also mu 1055.
[1] Likewise in Photius (mu449 Theodoridis) and elsewhere; and cf. Hesychius s.v. mikrou= (mu1345).
[2] That is, mikrou= by itself, without the dei=n, can mean 'almost', as is illustrated in the quotation that follows; cf. LSJ s.v. mikro/s, and Smyth §1399.
[3] Eunapius fr. 34 FHG (4.27); cf. Blockley, p. 51. The same passage is quoted more extensively at mu 1048 (q.v. for further commentary), there identifying Persian proconsul Asiae Hormisdas (Hormizd; cf. mu 230 (end)) as the subject of the participial phrase (Jones, et al., s.v. Hormisdas 3, p. 443; Blockley, p. 138).