Stable's opening tack room — it's slow (6,4)
I believe the answer is:
snails pace
'it's slow' is the definition.
(moving at a snail's pace is moving slowly)
'stable's opening tack room' is the wordplay.
'opening' says to take the initial letters.
'tack' becomes 'nail' (I've seen this before).
'room' becomes 'space' (I've seen this before).
The initial letter of 'stables' is 's'.
's'+'nail'+'space'='SNAILS PACE'
(Other definitions for snails pace that I've seen before include "making slow progress" , "no rush" , "Slow rate of progress" , "at a very low rate" , "Very slow speed" .)