Time to replace learner in sluggish pack (4)
I believe the answer is:
stow
'sluggish pack' is the definition.
'stow' can be an answer for 'pack' (thesaurus). I am not sure about the 'sluggish' bit.
'time to replace learner in' is the wordplay.
'time' becomes 't'.
'to replace learner' becomes 'sow' (I can't justify this - if you can you should believe this answer much more).
'in' is an insertion indicator.
't' inserted inside 'sow' is 'STOW'.
Can you help me to learn more?
(Other definitions for stow that I've seen before include "Put tidily away" , "- - -on-the-Wold" , "Pack (object) away" , "Keep under hatches" , "Lay up, pack away" .)