Working blouse on either sex is doing very well (2,3,4)

'is doing very well' is the definition.

'working blouse on either' is the wordplay.
'working' becomes 'on'.
'blouse' becomes 'top'.
'on' means one lot of letters go next to another.
'either' becomes 'form' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'on'+'top'+'form'='ON TOP FORM'

'sex' is the link.
I am not very happy about this link. It may belong to another bit of the clue.