The Blues retrospectively send in crack team (7)
I believe the answer is:
sadness
'the blues' is the definition.
(I've seen this before)
'retrospectively send in crack team' is the wordplay.
'retrospectively' says the letters should be written backwards.
'in' means one lot of letters goes inside another.
'crack team' becomes 'sas' (Special Air Service, crack team of soldiers).
'send' put into 'sas' is 'ssendas'.
'ssendas' back-to-front is 'SADNESS'.
(Other definitions for sadness that I've seen before include "Sorrowful quality" , "Depression" , "Sorrow, unhappiness" , "Sorrowfulness" , "no joy there" .)