Think around 150 needed to show strength (6)
I believe the answer is:
muscle
'strength' is the definition.
(muscle can mean strength or force)
'think around 150' is the wordplay.
'think' becomes 'muse' (to muse on something is to think about it).
'around' is an insertion indicator.
'150' becomes 'CL' (Roman numerals).
'muse' going around 'cl' is 'MUSCLE'.
'needed to show' is the link.
(Other definitions for muscle that I've seen before include "It can contract in the body" , "Body tissue that effects movement" , "Physical power" , "A contractile organ of the body" , "for bearing around?" .)