Branch line with some storage capacity (4)
I believe the answer is:
limb
'branch' is the definition.
(I've seen this in another clue)
'line with some storage capacity' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'with' says to put letters next to each other.
'some' becomes 'i' (I am not sure about this - if you are sure you should believe this answer much more).
'storage capacity' becomes 'MB' (abbreviation for megabytes).
'l'+'i'+'mb'='LIMB'
Can you help me to learn more?
(Other definitions for limb that I've seen before include "A supporter" , "Branch - arm - leg" , "Substantial branch from a tree trunk" , "arm, perhaps" , "Jointed appendage" .)