Strong tissues with boring functions (6)
I believe the answer is:
sinews
'strong tissues' is the definition.
'with boring functions' is the wordplay.
'with' becomes 'w' (abbreviation).
'boring' indicates putting letters inside (some letters must 'bore' a hole into other letters).
'functions' becomes 'sines' (sine is a kind of function**).
'w' placed into 'sines' is 'SINEWS'.
(Other definitions for sinews that I've seen before include "They connect muscle to bone" , "Strength" , "Tissue connecting muscles to bones" , "Tissues connecting muscles to bones" , "Tendons, ligaments" .)