Marriageable perhaps in blue (6)
I believe the answer is:
nubile
'marriageable' is the definition.
(I know that marriageable can be written as nubile)
'perhaps in blue' is the wordplay.
'perhaps' indicates an anagram.
'in'+'blue'='inblue'
'inblue' anagrammed gives 'NUBILE'.
(Other definitions for nubile that I've seen before include "Sexually mature" , "Marriageable, physically attractive" , "Sexually attractive" , "fit for match" , "Of girls or women who are eligible to marry" .)