Unlimited trading hit rock bottom (5)
I believe the answer is:
nadir
'rock bottom' is the definition.
(I've seen this before)
'unlimited trading hit' is the wordplay.
'unlimited' indicates the central letters (discard the edges or 'limits' of the word).
'hit' is an anagram indicator.
The centre of 'trading' is 'radin'.
'radin' is an anagram of 'NADIR'.
(Other definitions for nadir that I've seen before include "Drain to lowest level" , "Low-water mark" , "Low point" , "All-time low" , "Point opposite the zenith" .)