Measureless chasm in area near steamship (5)
I believe the answer is:
abyss
'measureless chasm' is the definition.
(I've seen this before)
'area near steamship' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'near' becomes 'by' (near means close by).
'steamship' becomes 'ss' (nautical abbreviation).
'a'+'by'+'ss'='ABYSS'
'in' acts as a link.
(Other definitions for abyss that I've seen before include "depths of sea" , "Very large and deep chasm" , "Hell; hole" , "Deep hole, seems bottomless" , "deep depression" .)