Mystery source of growth in a pit in recession (6)
I believe the answer is:
enigma
'mystery' is the definition.
(I know that mystery can be written as enigma)
'source of growth in a pit in recession' is the wordplay.
'source of' indicates taking the first letters (the start - or source - of the word).
'in' means one lot of letters goes inside another.
'pit' becomes 'mine' (mining pit).
'in recession' shows that the letters should be reversed in order (letters recede or go back).
The initial letter of 'growth' is 'g'.
'a'+'mine'='amine'
'g' placed within 'amine' is 'amgine'.
'amgine' backwards is 'ENIGMA'.
(Other definitions for enigma that I've seen before include "German military cipher machine" , "Difficult problem" , "Coding device" , "There's a mystery in game" , "Puzzling person/thing" .)