Severe depression doing head in (5)
I believe the answer is:
rough
'severe' is the definition.
(thesaurus)
'depression doing head in' is the wordplay.
'depression' becomes 'trough' (trough is a kind of depression).
'doing head in' means to remove the first letter (the word\'s head is removed).
'trough' with its initial letter taken off is 'ROUGH'.
(Other definitions for rough that I've seen before include "Not smooth, even or regular" , "Delinquent" , "scratchy" , "Very choppy" , "Uneven, harsh" .)