Stair hamsters perhaps will climb (4)
I believe the answer is:
step
'stair' is the definition.
(I know that stair can be written as step)
'hamsters perhaps will climb' is the wordplay.
'hamsters perhaps' becomes 'pets'.
'will climb' is a reversal indicator (down clues going up).
'pets' back-to-front is 'STEP'.
(Other definitions for step that I've seen before include "Ladder rung; pace" , "short distance" , "Place foot on as with a pest" , "Pace - part of a procedure" , "Put foot on a pest" .)