Most desperate little daughter, one needing repose (6)
I believe the answer is:
direst
'most desperate' is the definition.
(dire can mean desperate or terrible)
'little daughter one needing repose' is the wordplay.
'little' indicates taking the first letters.
'one' becomes 'i' (Roman numeral).
'needing' means one lot of letters go next to another.
'repose' becomes 'rest' (synonyms).
The first letter of 'daughter' is 'd'.
'd'+'i'+'rest'='DIREST'
(Other definitions for direst that I've seen before include "Most horrific" , "far from ideal" .)