Strength of current in all directions (5)
I believe the answer is:
sinew
'strength' is the definition.
(I know that sinew is a type of strength)
'current in all directions' is the wordplay.
'current' becomes 'I' (physics symbol).
'in' indicates putting letters inside.
'all directions' becomes 'snew' (compass directions).
'i' inserted inside 'snew' is 'SINEW'.
'of' acts as a link.
(Other definitions for sinew that I've seen before include "Tendon, for example" , "Tendon - ligament" , "fibre" , "Strength; tendon" , "It indicates strength" .)