One who speculates over blocking specific part of stair? (9)
I believe the answer is:
theoriser
'one who speculates' is the definition.
(I've seen this before)
'over blocking specific part of stair?' is the wordplay.
'over' becomes 'o' (cricket abbreviation**).
'blocking' indicates putting letters inside.
'specific' becomes 'the' ('the' is used to refer to a specific item).
'part of stair?' becomes 'riser' ('riser' is part of 'stair').
'the'+'riser'='theriser'
'o' put within 'theriser' is 'THEORISER'.
(Other definitions for theoriser that I've seen before include "he thinks" , "One who reasons" , "speculator" , "one who speculates" , "One who has convictions" .)