Low-cost airline once involving Moss — surprisingly, it's unprofitable (9)

'it's unprofitable' is the definition.
(I know that lossmakers are unprofitable)

'low-cost airline once involving moss surprisingly' is the wordplay.
'low-cost airline once' becomes 'laker' (Laker Airways, former airline).
'involving' means one lot of letters goes inside another.
'surprisingly' indicates anagramming the letters (a surprising ordering of the letters).
'moss' anagrammed gives 'ossm'.
'laker' enclosing 'ossm' is 'LOSSMAKER'.